Cement calculator finds bags, sand, gravel, and volume using slab volume = length × width × depth or cylinder volume = π × radius² × depth, then applies waste and mix ratio.
Cement and Concrete Are Not the Same Thing — and That Confusion Costs Money
Cement is an ingredient. Concrete is the finished material. Ask someone how much cement they need for a patio slab and most will describe a concrete project — which means the answer depends entirely on whether they’re buying pre-mixed bags from a hardware store or sourcing Portland cement powder to mix themselves with sand and gravel. These two paths produce completely different estimates, and conflating them is the single most common purchasing mistake on small-to-medium concrete jobs.
This calculator handles both paths explicitly. Switch between Custom Portland Cement Mix and Pre-mixed Concrete Bags using the Calculation Mode selector, and the outputs — and the math behind them — change accordingly.
How the Two Modes Calculate Differently
Pre-mixed bag mode is the simpler path. The calculator computes your pour volume (with waste), then divides by the published yield of your selected bag size. US bags are sized at 80 lb (0.60 cu ft yield), 60 lb (0.45 cu ft), 50 lb (0.375 cu ft), and 40 lb (0.30 cu ft). Metric bags run from 20 kg to 40 kg with corresponding cubic metre yields. Bags are always rounded up — you can’t buy a fraction of a bag — and the third output card shows alternative bag size quantities so you can compare what’s cheapest or most practical at your supplier.
Custom Portland cement mix mode involves a step that catches a lot of first-time mixers off guard. The calculator multiplies your target pour volume by 1.5 before dividing out ingredient proportions. That factor accounts for the fact that dry bulk materials — cement powder, sand, and gravel — contain air voids between particles. When you combine and add water, those voids collapse and the mix consolidates. To end up with, say, 1 cubic foot of cured concrete, you need roughly 1.5 cubic feet of dry ingredients going in. Skip this factor and you’ll consistently run short.
From the inflated dry volume, the calculator applies your chosen mix ratio. The standard 1:2:3 mix (cement:sand:gravel) means cement takes 1/6 of the dry volume, sand takes 2/6, and gravel takes 3/6. The required cement volume is then divided by your bag’s yield to produce a bag count. Sand and gravel quantities come out in cubic yards (or cubic metres when your length input is metric) so you can order aggregate by the yard or check how many bulk bags you need.
Both shapes — slab and cylinder — feed into the same volume calculation after geometry. Slabs use length × width × depth. Cylinders (footings, columns, fence post holes) use π × radius² × depth, with the width input automatically disabled since diameter alone defines the cross-section.
Why You Almost Always Need More Than You Think: The Waste Factor
The default waste factor is 5%, and for a clean formed slab pour on flat ground it’s reasonable. But several scenarios push that number higher, and the calculator’s waste field exists precisely because no textbook figure covers every site condition.
Uneven subgrade is the biggest culprit — a sub-base that’s nominally 4 inches but varies between 3.5 and 5 inches across the pour area means your actual volume exceeds the calculated one. Overfilling forms slightly before screeding also burns material. For cylindrical footings in hand-dug holes, soil cave-in and irregular walls routinely add 10–15% beyond the theoretical cylinder volume. Sloped sites, textured formwork, and cold-weather placements (where concrete stiffens faster) all benefit from a higher waste allowance.
For post hole footings or any project where individual pours are small, round up aggressively — running out mid-footing on a deck post means a cold joint in a structural element. For large ready-mix truck orders, being 0.5 yards over is far less costly than being 0.5 yards short and having to order a second partial truck at minimum-load surcharge rates.
Worked Example: A Detached Garage Apron Slab
A homeowner was pouring a concrete apron in front of a detached garage — 14 feet wide, 5 feet deep, 4 inches thick. They were mixing from scratch using bagged Portland cement and bulk aggregate ordered from a local supplier.
Inputs entered into the calculator:
- Shape: Slab
- Mode: Custom Portland Cement Mix
- Length: 14 ft
- Width: 5 ft
- Depth: 4 in
- Waste Factor: 10% (slightly high to account for uneven base along the garage door threshold)
- Mix Ratio: Standard Strong Mix (1:2:3)
- Cement Bag: 94 lb (US Standard)
Raw volume: 14 × 5 × (4/12) = 23.33 cu ft. With 10% waste: 25.67 cu ft. Applying the 1.5× dry volume factor gives 38.5 cu ft of dry ingredients. Cement’s share at 1/6: 6.42 cu ft — divided by the 94 lb bag’s 1.0 cu ft yield, rounded up: 7 bags of Portland cement. Sand came out at 0.48 cu yd and gravel at 0.71 cu yd.
The homeowner ordered 8 bags to have one spare (a sensible call for a small batch mix), confirmed the aggregate volumes with their supplier, and had materials left over rather than running short. The third output card also showed what the equivalent would have been in pre-mix bags — 43 bags of 80 lb pre-mix — which made the custom mix approach obviously more economical at bulk aggregate prices.
Frequently Asked Questions
When I switch from Slab to Cylinder mode, the Width field goes grey. Did I break something?
No — that’s the intended behaviour. A cylinder is fully defined by diameter and depth, so the width input is automatically disabled when Cylinder is selected. The Length field relabels itself as “Diameter” to clarify what you’re entering. If you switch back to Slab mode, the width field re-enables and resumes calculating normally. Entering a value in a greyed-out field has no effect on the result.
Each dimension has its own unit selector. Can I mix units — say, feet for length and inches for depth?
Yes, and that’s intentional. Depth is most naturally entered in inches for most slab work, while length and width are in feet — so the calculator defaults to exactly that combination. Each field converts independently to feet internally before volume is calculated. The ft & in and m & cm dual-entry modes split a single dimension into two input boxes, which is useful when working from architectural drawings that specify, say, 2 ft 6 in rather than 2.5 ft. Switching unit modes while a value is already entered will attempt to convert the existing value into the new unit automatically.
What does the calculator do if I enter zero or a negative number for any dimension?
Any dimension of zero or below is treated as invalid — the outputs clear and a data-required message appears. A zero dimension would produce zero volume, which generates meaningless material quantities. Negative dimensions are physically impossible. The waste factor is the one field that accepts zero legitimately: entering 0% gives you exact theoretical volume with no buffer, which is valid if you’re doing a quick estimate and want to apply your own margin manually.
In Custom Mix mode, does switching between the 94 lb, 50 kg, and 40 kg cement bags change the sand and gravel quantities?
No — aggregate quantities are determined entirely by the mix ratio and target volume, not by bag size. The cement bag selector only affects the bag count and weight shown for cement. Sand and gravel volumes are fixed once shape, dimensions, and mix ratio are set. What does shift is whether the output displays in US or metric units: selecting a metric cement bag (50 kg or 40 kg) causes the cement weight and volume rows to display in kilograms and cubic metres rather than pounds and cubic feet.
The pre-mixed bag mode shows “Alternative Bag Sizes” in the third card. Where do those yields come from?
The calculator uses the industry-standard published yields for each bag size: 80 lb bags yield approximately 0.60 cubic feet, 60 lb bags yield 0.45 cubic feet, 50 lb bags yield 0.375 cubic feet, and 40 lb bags yield 0.30 cubic feet. These are the figures printed on Quikrete and Sakrete packaging and widely used in trade estimating. The alternative bag card lets you instantly compare quantities across sizes without re-entering dimensions — useful when your supplier is out of one size, or when smaller bags are on sale and you want to know the quantity difference before deciding.