What Is Cement?
Cement is the binder, not the finished material. It is a fine gray powder that reacts chemically with water rather than drying out, and the hydrates that reaction forms are what glue sand and stone into a solid mass. Everything else in a concrete mix is inert filler held in place by what the cement does.
Cement Is Not Concrete
Concrete is the product; cement is one ingredient in it. The American Cement Association puts cement at 10 to 15 percent of a concrete mix by volume, the rest being sand, coarse aggregate, water and entrained air. That range is a useful sanity check on this calculator: in a 1:2:4 mix cement is one part in seven, or 14.3 percent of the dry volume, at the top of the range. A 1:3:6 mix is one part in ten, exactly 10 percent. A 1:4:8 mix is one part in thirteen, 7.7 percent — leaner than the association's range because it is not really structural concrete.
The distinction matters at the merchant's counter. A bag labeled "concrete mix" already contains sand and stone and needs only water. A bag of portland cement contains cement alone, and you buy the sand and aggregate separately. This calculator counts the second kind. If you are buying pre-blended concrete mix, none of the bag figures on this page apply.
Hydraulic Cement and Why It Sets Underwater
"Hydraulic cement" is the broader term. The American Cement Association defines it as a material that forms solid hydrates on reacting with water, and can do so under water. That property is what separates modern cement from the lime mortars that preceded it: a footing poured into a trench with water in the bottom still gains strength, which is why hydraulic cement replaced everything else for foundation work.
Portland cement is the common hydraulic cement, manufactured to ASTM C150 in four basic types — Type I for general use, Type II for moderate sulfate exposure, Type III for high early strength, and Type V for severe sulfate exposure. Blended cements combine portland clinker with limestone, blast-furnace slag or pozzolans such as fly ash, and portland-limestone cement has outshipped straight portland cement in the US since mid-2023.
For the arithmetic on this page the type does not matter: the calculator treats every 50 kg bag as 1.25 cubic feet regardless of what is in it. Type changes set time, heat of hydration, sulfate resistance and early strength, none of which appear anywhere in a volumetric ratio.
How Do You Calculate Cement Quantity?
Four multiplications and a division. Nothing in the chain is difficult; the two places people go wrong are entering thickness in the wrong unit and forgetting the dry-volume step entirely.
What You Need Before You Start
- Area of the pour, in square feet or square meters. Length × width for a rectangle; split an L-shape into rectangles and add them.
- Thickness, in feet or meters. There is no inches or millimeters option, so convert before you type.
- Mix ratio — 1:2:4, 1:3:6 or 1:4:8, read as cement : sand : coarse aggregate and measured by volume, not by weight.
That is the entire input list. There is no field for water, no field for cement type, and no field for wastage. Each of those is a decision you make around the result rather than inside it, and the sections below deal with all three.
Step by Step
- Step 1 — Multiply area by thickness. In feet and square feet that gives cubic feet directly. Metric entries are converted first (1 m² = 10.764 ft², 1 m = 3.281 ft), so the tool always works internally in cubic feet.
- Step 2 — Multiply by 1.54. This is the dry volume: the volume of loose, unmixed material required to end up with that much placed concrete.
- Step 3 — Add the parts of the ratio. 1:2:4 totals 7 parts, 1:3:6 totals 10, and 1:4:8 totals 13.
- Step 4 — Divide the dry volume by the total parts. That gives the volume of one part, and cement is one part in every ratio offered here.
- Step 5 — Divide the cement volume by 1.25 cubic feet, the assumed volume of a 50 kg bag. Round the answer up when you buy.
Steps 3 and 4 are the only place the ratio does any work. Sand and aggregate never enter the bag count — they take the other parts of the same dry volume, which is why you can pull them out of a finished result without running anything again.
Entering Thickness in Feet or Meters
Slabs are specified in inches or millimeters and this field takes feet or meters, so a conversion has to happen before you type. Get it wrong and the answer is out by a factor of twelve or a thousand, in a way that looks entirely plausible on screen.
| Nominal thickness | Enter in the feet field | Enter in the meters field |
|---|---|---|
| 3 in | 0.25 | 0.076 |
| 4 in | 0.33 | 0.102 |
| 5 in | 0.42 | 0.127 |
| 6 in | 0.50 | 0.152 |
| 8 in | 0.67 | 0.203 |
| 75 mm | 0.246 | 0.075 |
| 100 mm | 0.328 | 0.100 |
| 150 mm | 0.492 | 0.150 |
| 200 mm | 0.656 | 0.200 |
Type 4 instead of 0.33 for a 10 × 12 ft slab and the calculator reports 480.00 cu ft and 84.5 bags rather than 39.60 cu ft and 7.0 bags. The volume line is the guard against that, so read it before you look at the bags.
Cement Formula
The whole chain, in the order the calculator runs it.
Wet volume (cu ft) = area × thickness Dry volume (cu ft) = wet volume × 1.54 Cement volume (cu ft) = dry volume ÷ total parts Cement bags = cement volume ÷ 1.25 Total parts: 1:2:4 → 7 1:3:6 → 10 1:4:8 → 13 Metric input: 1 m² = 10.764 ft² 1 m = 3.281 ft
Collapsed to a single line, bags = area × thickness × 1.54 ÷ parts ÷ 1.25, which simplifies to area × thickness × 1.232 ÷ parts. That one constant, 1.232, is the whole tool once you know which ratio you are on.
Why the 1.54 Dry-Volume Factor
A cubic foot of loose sand is mostly sand with air between the grains, and a cubic foot of coarse aggregate has far larger voids again. Mix everything with cement and water, place it and compact it, and the fine material migrates into the gaps between the coarse. The finished concrete occupies noticeably less space than the ingredients did sitting in separate heaps.
1.54 is the trade allowance for that loss: 1.54 cubic feet of dry material for every cubic foot of placed concrete, a 54 percent uplift. It is a convention rather than a measured constant for your particular materials — sand bulking, aggregate grading and how hard the mix is compacted all move it, and site figures between roughly 1.50 and 1.57 are all in use. This calculator fixes it at 1.54 for all three ratios.
Leave the step out and the order comes up 35 percent short. On 100 cubic feet of 1:2:4 concrete the calculator returns 17.6 bags; the same sum without the factor gives 11.4. That gap is the most common reason a hand-worked estimate runs out of cement halfway through a pour.
Why 1.25 Cubic Feet per Bag
Cement is sold by mass and batched by volume, so the two have to be bridged by a bulk density. Treating a 50 kg bag as 1.25 cubic feet implies a loose density of about 1,413 kg per cubic meter, and it has the practical advantage of matching a standard gauge box: 12 × 12 × 15 inches is exactly 1.25 cubic feet, so one box is one part.
The textbook density more often quoted is 1,440 kg/m³, which puts a 50 kg bag at 1.226 cubic feet. Adopting it would raise every bag figure on this page by 1.9 percent — the worked example below moves from 6.97 bags to 7.10. That difference disappears the moment you round up to whole bags, which is why the looser figure is the safe one to plan with.
Worked Example: A 10 × 12 ft Slab
A shed slab 10 ft by 12 ft, 4 inches thick, batched 1:2:4. Every figure below is either the calculator's own output or one arithmetic step away from it.
The Inputs
- Area Unit: Square Feet
- Total Area: 120 (10 × 12)
- Thickness Unit: Feet
- Concrete Thickness: 0.33 (4 inches)
- Concrete Mix Ratio: 1:2:4 (Strong Mix)
- Calculator output
- Concrete Volume: 39.60 cu ft · Mix Ratio: 1:2:4 · Cement Required: 7.0 bags
The Arithmetic, Line by Line
- Wet volume: 120 ft² × 0.33 ft = 39.60 cu ft, which is 1.12 m³ or 1.47 cubic yards.
- Dry volume: 39.60 × 1.54 = 60.98 cu ft.
- Total parts in 1:2:4: 1 + 2 + 4 = 7.
- Cement volume: 60.98 ÷ 7 = 8.71 cu ft.
- Bags: 8.71 ÷ 1.25 = 6.97, displayed as 7.0.
- Cement mass: 7 × 50 kg = 350 kg, which works out at 312 kg of cement per cubic meter of concrete.
Enter 0.3333 rather than 0.33 and the volume reads 40.00 cu ft while the bag count stays at 7.0. The third decimal on thickness never changes what you buy, so there is no point being precious about it.
Getting Sand and Aggregate from the Same Dry Volume
The calculator returns cement bags only. Sand and aggregate come out of the dry volume you already have, using their own parts of the same ratio — no second calculation needed.
- Cement: 60.98 × 1/7 = 8.71 cu ft (0.25 m³), the 7.0 bags above.
- Sand: 60.98 × 2/7 = 17.42 cu ft (0.49 m³).
- Aggregate: 60.98 × 4/7 = 34.85 cu ft (0.99 m³).
- The three add back to 60.98 cu ft, which is the check that you used the right parts.
Sand and stone are sold by loose volume or by weight, and both bulk up when damp and settle again in transit, so treat these as order quantities rather than exact deliveries. For the coarse fraction it is worth cross-checking the figure against the Gravel Calculator.
Cement Chart: Bags by Volume and Mix
Reference figures, every one produced by running this calculator rather than by rounding a rule of thumb. Bag counts are for 50 kg bags throughout.
Bags per Unit of Concrete
| Mix ratio | Bags per 100 cu ft | Bags per cubic yard | Bags per cubic meter | Cement content |
|---|---|---|---|---|
| 1:2:4 | 17.6 | 4.8 | 6.2 | 310 kg/m³ |
| 1:3:6 | 12.3 | 3.3 | 4.4 | 220 kg/m³ |
| 1:4:8 | 9.5 | 2.6 | 3.3 | 165 kg/m³ |
Read the last column as the honest label on each mix. Moving from 1:2:4 to 1:4:8 takes cement content from 310 kg/m³ down to 165 — that is not a small economy on the same material, it is a different material, and strength and durability fall with it.
How Much Concrete One Bag Places
Invert the table and you get the figure worth carrying in your head on site: what a single bag is worth once mixed, and what goes in the box alongside it.
| Mix ratio | Concrete placed per 50 kg bag | In cubic meters | Per bag: sand | Per bag: aggregate |
|---|---|---|---|---|
| 1:2:4 | 5.68 cu ft | 0.161 m³ | 2.50 cu ft | 5.00 cu ft |
| 1:3:6 | 8.13 cu ft | 0.230 m³ | 3.75 cu ft | 7.50 cu ft |
| 1:4:8 | 10.53 cu ft | 0.298 m³ | 5.00 cu ft | 10.00 cu ft |
The last two columns describe a gauge box loaded one bag at a time: tip the bag in, add the listed volumes of sand and stone, and the proportions come out right without anything being weighed. Because the box is 1.25 cu ft, a 1:2:4 batch is one bag plus two boxes of sand and four of aggregate.
Slab Chart, per 100 ft²
Concrete volume and bags for every 100 square feet of slab. The relationship is linear in area, so a 250 ft² slab is 2.5 times the row you are reading.
| Slab thickness | Enter (feet) | Concrete per 100 ft² | 1:2:4 | 1:3:6 | 1:4:8 |
|---|---|---|---|---|---|
| 3 in | 0.25 | 25.00 cu ft | 4.4 | 3.1 | 2.4 |
| 4 in | 0.33 | 33.00 cu ft | 5.8 | 4.1 | 3.1 |
| 5 in | 0.42 | 42.00 cu ft | 7.4 | 5.2 | 4.0 |
| 6 in | 0.50 | 50.00 cu ft | 8.8 | 6.2 | 4.7 |
| 8 in | 0.67 | 67.00 cu ft | 11.8 | 8.3 | 6.3 |
Slab Chart, per 10 m²
The same table for metric entry. Volumes here are the calculator's cubic-foot output converted back to cubic meters, which is why 10 m² at 100 mm lands on exactly 1.00 m³.
| Slab thickness | Enter (meters) | Concrete per 10 m² | 1:2:4 | 1:3:6 | 1:4:8 |
|---|---|---|---|---|---|
| 75 mm | 0.075 | 0.75 m³ | 4.7 | 3.3 | 2.5 |
| 100 mm | 0.10 | 1.00 m³ | 6.2 | 4.4 | 3.3 |
| 150 mm | 0.15 | 1.50 m³ | 9.3 | 6.5 | 5.0 |
| 200 mm | 0.20 | 2.00 m³ | 12.4 | 8.7 | 6.7 |
Typical Jobs, Start to Finish
| Job | Inputs | Concrete volume | Cement bags |
|---|---|---|---|
| Shed slab, 10 × 12 ft, 4 in, 1:2:4 | 120 ft² · 0.33 ft | 39.60 cu ft | 7.0 |
| Shed base, 8 × 10 ft, 4 in, 1:3:6 | 80 ft² · 0.33 ft | 26.40 cu ft | 3.3 |
| Driveway, 400 ft², 5 in, 1:2:4 | 400 ft² · 0.42 ft | 168.00 cu ft | 29.6 |
| Strip footing, 60 ft², 9 in, 1:3:6 | 60 ft² · 0.75 ft | 45.00 cu ft | 5.5 |
| Blinding layer, 200 ft², 3 in, 1:4:8 | 200 ft² · 0.25 ft | 50.00 cu ft | 4.7 |
| Garage floor, 20 m², 150 mm, 1:2:4 | 20 m² · 0.15 m | 105.95 cu ft | 18.6 |
| Garden path, 12 m², 100 mm, 1:3:6 | 12 m² · 0.10 m | 42.38 cu ft | 5.2 |
| Workshop floor, 50 m², 100 mm, 1:2:4 | 50 m² · 0.10 m | 176.58 cu ft | 31.1 |
The footing row is the one to notice. Sixty square feet sounds like nothing next to a 400 ft² driveway, but at nine inches deep it still takes 45 cubic feet of concrete. Depth moves the number as hard as area does, and trenches are almost always deeper than the drawing says.
How to Read Your Result
Three figures come back: the concrete volume in cubic feet, the ratio echoed back to you, and the bag count to one decimal. Each is doing a different job.
Start With the Volume, Not the Bags
The volume line is the error trap, and it is the only part of the result you can check against the thing in front of you. A 10 × 12 ft slab at 4 inches is about 40 cu ft; a 400 ft² driveway at 5 inches is 168. If the volume is ten or a hundred times what the pour looks like, a unit went in wrong, and the bag count is wrong in exactly the same proportion without looking wrong on its own.
The volume is also the number you would quote a supplier, once it is out of cubic feet. The slab above is 1.47 cubic yards or 1.12 m³; the driveway is 6.22 cubic yards or 4.76 m³. Nobody sells concrete in cubic feet.
Rounding and Wastage
The bag figure carries a decimal because the arithmetic does, not because anyone sells 0.6 of a bag. Round up — and round up after adding a wastage allowance, not before.
- 7.0 bags + 5% = 7.4 → buy 8
- 29.6 bags + 5% = 31.1 → buy 32
- 18.6 bags + 10% = 20.5 → buy 21
Five percent covers ordinary spillage, a formwork edge that bows out under the weight, and a sub-base sitting a little lower in places than the string line claims. Ten is more realistic over rough ground or in an unformed trench. Cement stores badly once the bag is open and worse once it has been damp, so a spare bag is only cheap insurance if it can go back unopened.
50 kg Bags and 94 lb Sacks
The bag count assumes a 50 kg bag, the standard unit across metric markets. The US trade unit is the 94 lb sack — the convention being that a sack of portland cement holds close to one cubic foot — and the two are not interchangeable on a purchase order.
- 7.0 bags = 350 kg = 772 lb = 8.2 sacks of 94 lb → buy 9
- 29.6 bags = 1,480 kg = 3,263 lb = 34.7 sacks → buy 35
- 5.5 bags = 275 kg = 606 lb = 6.4 sacks → buy 7
The shortcut is to multiply the bag figure by 1.17 and round up. Working the other way, a 94 lb sack is 42.6 kg, so it counts as 0.85 of the bag this calculator uses.
What the Ratio Actually Buys You
The three options are nominal mixes, fixed by parts rather than designed to reach a strength. The naming convention comes from the Indian Standard nominal-mix table used across South Asia and much of the Gulf, where 1:2:4 is grade M15, 1:3:6 is M10 and 1:4:8 is M7.5 — the number being the characteristic cube strength in megapascals at 28 days.
In US units those cube strengths are roughly 2,180, 1,450 and 1,090 psi. American practice specifies cylinder strength (f′c) instead, which comes out lower than cube strength for the same concrete, so none of the three should be read as satisfying a US structural specification just because the megapascal figure looks adequate.
The practical reading: 1:2:4 for anything that carries load — slabs, beams, columns, reinforced footings. 1:3:6 for mass concrete and unreinforced footings. 1:4:8 for blinding, levelling courses and fill beneath a real slab, where the concrete separates the structure from the ground rather than holding anything up.
Dropping from 1:2:4 to 1:3:6 cuts cement by exactly 30 percent, because 7 parts become 10; going to 1:4:8 cuts it by 46 percent, 7 parts becoming 13. On the 10 × 12 ft slab that is 7.0 bags, 4.9 bags and 3.8 bags for the identical 39.60 cu ft of concrete. The saving is real, and so is what you give up for it.
Limits: When This Does Not Apply
Four places where the bag count stops being the number to trust.
A Nominal Mix Is Not a Designed Mix
Proportioning by volume assumes the sand and stone behave the way the convention expects. Damp sand bulks — the same mass occupies substantially more space at site moisture content, commonly a fifth to nearly a third more, an effect documented in every concrete-technology text — so a gauge box of damp sand delivers less sand than the ratio calls for and the batch comes out cement-rich without being any stronger for it. Aggregate that is dirty, poorly graded or undersized pushes the result the other way.
Where strength has to be demonstrated — anything a building official signs off, anything reinforced and load-bearing — concrete is specified by compressive strength and proportioned by a mix design against the actual materials. ACI 332, the US code for residential concrete, covers footings, foundation walls and slabs-on-ground on exactly that basis. A ratio picked from a dropdown is not a substitute for it, and this calculator does not pretend otherwise.
Water Is Not in This Calculation
There is no water field, and that is the largest single omission in any nominal mix. The water-cementitious ratio governs strength more directly than cement content does: the same 7 bags will produce sound concrete at a low ratio and poor concrete at a high one. Adding water on site to loosen a stiff mix is the classic route from a correct bag count to weak concrete, and NRMCA publishes an entire information sheet, CIP 26, on that one failure.
Nothing here tells you how much water to add. Take that from a mix design, the bag instructions or a slump target, and treat any water added after batching as a strength reduction you consciously chose.
Flat Pours of Constant Depth Only
Area × thickness describes a slab, a path, a blinding layer, or a footing of constant depth. It does not describe a column, a stepped footing, a thickened slab edge, a plinth, or anything poured on ground that dips. For a varying section, split the pour into constant-depth parts, run each one, and add the bag counts.
The tool also assumes the sub-base is exactly where you believe it is. A nominal 4-inch slab over ground sitting an inch low through the middle is nearer 4.5 inches on average — about an eighth more concrete, and an eighth more cement, before a single measurement has been mistyped.
When to Stop Mixing and Order a Truck
Hand batching stops making sense somewhere between one and two cubic yards. The 10 × 12 ft slab is 1.47 cubic yards and 7 bags — a long day with a mixer and two people, but achievable. The 400 ft² driveway is 6.22 cubic yards and 29.6 bags, and that is a truck, because a slab that size has to be placed and finished as one continuous pour and hand batching cannot outrun the set.
The other reason to settle the volume first is that ready-mix is sold in cubic yards or cubic meters, never in bags, and suppliers charge for part loads — so the figure to take to a quote is the volume line, not the bag line. NRMCA's CIP 31 sheet covers what a supplier needs to hear when you order. For volume on its own, including the shapes this tool cannot describe, use the Concrete Calculator.