Cement Calculator
Bags of cement plus sand and aggregate from slab size and mix ratio (1:2:4, 1:1.5:3, 1:3:6).
Dry volume ≈ 1.54 × wet volume. Cement assumed 94 lb/ft³. For ready-mix volume only, use the Concrete calculator.
This cement calculator converts a slab's dimensions and a mix ratio into bags of cement plus the sand and aggregate that go with them. Pick 1:2:4, 1:1.5:3, or 1:3:6, set the bag size you can actually buy, and it returns volume in cubic feet and cubic yards alongside the bag count.
Two ordinary mistakes account for most bad pours: buying cement when the job needs concrete, and sizing bags from the wet volume when bags are priced by dry volume. Both are handled below, along with the 5-10% waste factor the calculator deliberately leaves out.
How to calculate cement quantities
- Volume = length x width x depth, all in feet. A 10 x 10 ft slab at 4 in is 10 x 10 x 0.333 = 33.3 cu ft, or 1.23 cu yd.
- Dry volume = wet volume x 1.54 = 33.3 x 1.54 = 51.3 cu ft. The 54% covers the air voids that close once water and aggregate pack together.
- Split by the mix ratio. 1:2:4 has 7 parts, so cement is 51.3 x 1/7 = 7.33 cu ft.
- Convert to bags. One cubic foot of Portland cement weighs 94 lb, so 7.33 cu ft is 7.33 bags of 94 lb — or about 8.6 bags if you buy 80 lb.
- Sand = 51.3 x 2/7 = 14.7 cu ft. Aggregate = 51.3 x 4/7 = 29.3 cu ft.
- Add 5-10% for spillage and an uneven subgrade: buy 8 bags of 94 lb rather than 7.3.
Example
A 10 x 10 ft slab, 4 in thick, at 1:2:4 with 94 lb bags: 1.23 cu yd of concrete, 7.3 bags of cement, 14.7 cu ft of sand, and 29.3 cu ft of aggregate. Switch to 1:1.5:3 and cement jumps to 9.3 bags for the same slab — a stronger mix costs cement, not volume.
Cement, sand, and aggregate formula
Wet volume (cu ft) = L x W x D in feet. Dry volume = wet x 1.54. Cement = dry x 1 / (1+2+4). Bags = cement cu ft x 94 / bag weight in lb.
How to use this calculator
- Enter the slab or footing dimensions in feet, with depth in inches.
- Pick the mix ratio. 1:2:4 is the general-purpose default; 1:1.5:3 for structural work; 1:3:6 for blinding and non-structural fill.
- Set the bag size — 94 lb, 80 lb, or 50 lb.
- Read bags of cement first, then sand and aggregate in cubic feet.
- Order 5-10% extra. A spare bag stores for months if it stays dry, and a short pour cannot be fixed later.
Which mix ratio to use
A richer mix means more cement in the same volume of concrete, not more concrete. If you are ordering ready-mix instead, the plant has already chosen the ratio and only the cubic yards matter.
| Cement : sand : aggregate | Typical use | Cement needed | Sand / aggregate |
|---|---|---|---|
| 1 : 1.5 : 3 | Structural slabs, footings, beams — about 4,000 psi | 9.3 bags | 14.0 / 28.0 cu ft |
| 1 : 2 : 4 | General slabs, paths, shed bases — about 3,000 psi | 7.3 bags | 14.7 / 29.3 cu ft |
| 1 : 3 : 6 | Blinding, non-structural fill, mass concrete | 5.1 bags | 15.4 / 30.8 cu ft |
Same 10 x 10 ft slab at 4 in thick
Cement is not concrete
Cement is the grey powder — the glue. Concrete is cement plus sand, aggregate, and water. Cement on its own sets into a brittle, shrinking paste with almost no tensile strength, and it is not a substitute for concrete in any structural application.
The distinction matters when ordering. How many bags of cement for a 10 x 10 slab and how much concrete for a 10 x 10 slab have different answers, and only the second one can be delivered by a ready-mix truck.
The 1.54 factor, and why dry volume is bigger
Sand and aggregate arrive with air between the grains. Once water and cement fill those gaps, the same mass occupies less space — so a cubic foot of finished concrete needs about 1.54 cubic feet of loose dry material.
Sizing bags from the wet volume under-orders cement by roughly a third, which is the single most common arithmetic error on a self-mixed pour. Some references use 1.52 or 1.57 depending on aggregate shape and moisture; the difference is under 3%, well inside the waste factor.
Bag sizes: 94, 80, and 50 lb
US Portland cement is sold in 94 lb bags because that bag holds exactly one cubic foot. The 80 lb bag is 0.85 cu ft and the 50 lb bag is 0.53 cu ft, so a recipe written in bags is only portable if you also know the bag weight.
Metric markets sell 25 kg or 50 kg bags; 25 kg is about 0.55 cu ft. Always round up — cement is cheap relative to a failed pour.
What the calculator leaves out
- Water — roughly 0.45-0.55 lb per lb of cement, which is about 4-5 gallons for 7.3 bags.
- Waste — 5% for a formed slab, 10% for footings on uneven ground.
- Reinforcement — mesh or rebar is sized separately from the concrete volume.
- Sub-base and formwork — usually more material and labour than the concrete itself.
- Ready-mix minimums — many plants charge for a full cubic yard even if you need half.
Common mistakes
- Ordering cement only. Cement plus sand plus aggregate is concrete; cement alone is not.
- Sizing bags from the wet volume, which under-orders by about a third.
- Mixing by volume with a damp sand pile. Damp sand bulks up, so a shovel holds less material than it looks.
- Assuming the form depth is the slab depth. A 4 in slab over an uneven subgrade is often 5 in of concrete.
FAQ
How many bags of cement do I need for a 10 x 10 slab?
At 4 in thick that is 1.23 cu yd of concrete. With a 1:2:4 mix and 94 lb bags, about 7.3 bags — buy 8 to 9 with waste. With 80 lb bags, about 9.
What does 1:2:4 mean?
One part cement, two parts sand, four parts aggregate, measured by volume. It is the general-purpose mix at roughly 3,000 psi.
How much sand and gravel do I need per bag of cement?
In a 1:2:4 mix, each 94 lb bag takes about 2 cu ft of sand and 4 cu ft of aggregate. The calculator scales this to your slab.
Is cement the same as concrete?
No. Cement is the powder; concrete is cement, sand, aggregate, and water. A slab cannot be poured with cement alone.
How many 80 lb bags equal a 94 lb bag?
1.175. Seven 94 lb bags is about 8.2 bags of 80 lb, so round to 9.
How much water goes into the mix?
About 0.45-0.55 lb of water per lb of cement — roughly 4-5 gallons for 7.3 bags. More water makes it easier to place and weaker once cured.
How many cubic feet are in a 94 lb bag of cement?
Exactly one. That is why the bag weighs 94 lb: it is one cubic foot of Portland cement at its usual bulk density.
Can I use this for a footing or a column?
Yes, if you can express it as length x width x depth. For a round column, compute pi x r squared x height in feet and enter it as an equivalent rectangular volume.
How much extra should I order?
5% for a formed slab, 10% where the subgrade is uneven or the forms are rough. Leftover bags keep for months if stored dry.