Concrete Calculator

Estimate slab or cylinder concrete volume and whole bags using your product yield and allowance.

Estimate volume and whole bags for rectangular sections or solid cylinders. Enter the mixed-concrete yield from your actual product label. This quantity calculation does not specify structural thickness, strength or mix proportions.

Calculations stay in this browser. Measurements are not uploaded or saved. Your optional JSON download contains the active dimensions, product yield and assumptions.

This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.

Concrete Calculator converts measured rectangular or cylindrical quantities into volume and whole bags using your actual product yield and chosen allowance.

How to use it

1Choose metric or US units, a section shape and its measured dimensions.
2Enter identical-section count, mixed-concrete yield per bag and your allowance.
3Review whole bags and rounding surplus, then download the quantity report with its assumptions.

Example

Input: 2 × 3 m slab, 100 mm thick; 10% allowance; 20 L mixed-concrete yield per bag.
Output: 0.6 m³ base; 0.66 m³ required; 33 whole bags.

Example inputs demonstrate quantity arithmetic, not structural or product recommendations.

What happens to your data

Concrete dimensions and bag quantities are calculated on this device. They are not uploaded or saved; the optional JSON download includes only the active shape measurements, product yield and assumptions.

Last updated October 2026

A concrete quantity estimate has two separate decisions: how much mixed concrete the measured shape holds, and how many whole bags of a particular product produce that volume. Bag weight alone cannot answer the second question. This concrete calculator keeps both decisions visible. Enter a rectangular slab or a solid circular cylinder, supply the manufacturer's mixed-concrete yield per bag, and choose an extra allowance. The result shows volume before allowance, required volume, full bags and the extra yield introduced by bag rounding.

Use the identical-sections field when several sections have the same dimensions. Their volumes are added before bags are rounded, so the report represents a combined quantity estimate. Calculate different shapes separately if their dimensions vary. The metric and US customary modes use explicit units on every field; changing modes clears dimensional inputs so that 100 millimetres cannot silently become 100 inches.

How it works

For a rectangular slab, volume is length multiplied by width multiplied by thickness. Metric length and width are in metres, while thickness is in millimetres. In US customary mode the first two dimensions are feet and thickness is inches. For a solid cylinder, volume is π multiplied by the square of half the diameter, then multiplied by height. Diameter and cylinder height are both metres or both feet. These are geometric quantities: a hollow pipe, a post embedded inside a hole, an irregular excavation or a varying slab thickness is not silently modelled as a solid uniform shape.

The single-section quantity is multiplied by your whole-number count. Next, the chosen allowance multiplies that combined volume by 1 plus the percentage divided by 100. A zero allowance is accepted and is the initial setting. Entering 10% in an example illustrates arithmetic; it does not recommend 10% for an unknown site. Waste, over-excavation, uneven substrate, handling loss and practical product yield can change the final order. No hidden percentage is added.

Bag yield must be the mixed-concrete volume specified for the actual bag, entered as litres per bag in metric mode or cubic feet per bag in US mode. The program converts that yield into the same volume unit as the requirement, divides, and rounds up once to a whole-bag count. It also multiplies the bag count back by the stated yield. Purchased yield minus required volume is the calculated surplus from rounding, kept separate from your original allowance. A 20-litre yield does not mean a 20-kilogram bag; dry mass and mixed volume are different quantities.

Defined conversions use exactly 0.3048 metres per international foot, 12 inches per foot, 27 cubic feet per cubic yard and 1,000 litres per cubic metre. Decimal slab inputs and whole-bag decisions use fractions, so a binary floating-point artefact cannot turn a mathematically exact three-bag result into four. Cylinder geometry involves irrational π. The implementation bounds π to 50 decimal places and requires both bounds to give the same whole-bag result. Displayed cylinder volumes remain approximations. The JSON report records this arithmetic distinction.

Dimensions and yield must be positive plain decimals with no commas, exponent notation or more than six decimal places. A dimension may not exceed 10,000 metres equivalent; the combined required volume is capped at 1,000,000 m³ and the result at 1,000,000 bags. Identical sections range from one to 10,000, and allowance ranges from zero to 100%. These are software limits, not appropriate dimensions for a real project. Editing any field clears the previous result and download. Only the selected shape's dimensions enter the exported report.

Calculations run in the browser without uploading or persisting measurements. The optional JSON download contains your dimensions, count, product yield, allowance, conversions and assumptions. The estimate does not select a mix, prescribe water, specify structural thickness or reinforcement, calculate loads, confirm suitability or replace product instructions. Its purpose is to make an entered quantity calculation auditable.

Worked example: Convert a slab volume into whole bags using an explicit yield

Input: One rectangular section, 2 m × 3 m × 100 mm; 10% extra allowance; product yield 20 L per bag.
Result: 0.6 m³ before allowance; 0.66 m³ (660 L) required; 33 whole bags; 660 L stated purchase yield; 0 L surplus from bag rounding.

The thickness, allowance and 20 L yield are arithmetic examples, not a specification or a claim about a particular product.

Common use cases

Checking a rectangular quantity estimate against a supplier's mixed-concrete yield per bag
Combining identical measured sections before rounding the total bag purchase
Converting a measured concrete requirement between cubic metres, cubic feet and cubic yards
Recording the difference between chosen allowance and the extra quantity caused by full-bag rounding

Frequently asked questions

Can I enter the weight printed on a concrete bag as its yield?＋

No. Yield is the mixed volume produced by a bag under the product's stated conditions. A mass in kilograms or pounds cannot be entered as litres or cubic feet. Read the manufacturer's yield information for that product and bag size. If yield is unknown, this calculator cannot infer a trustworthy bag count.

Does the concrete bag calculator tell me how thick a slab should be?＋

No. Thickness is a measurement or specification you supply. The tool performs quantity arithmetic only and does not assess loads, reinforcement, soil, frost, foundations or compliance. The example's 100 mm is not a recommended construction detail.

What does the identical-sections field change?＋

It multiplies the volume of one entered shape before allowance and bag rounding. Three identical sections share one aggregate bag quantity. It does not reserve a separate rounded bag count for each pour, plan mixing batches or account for unusable material between work sessions.

Can I calculate concrete around an existing post?＋

The cylinder mode is solid and does not deduct a post or pipe. Using the full hole as a solid cylinder includes that displacement and may overestimate its geometric concrete volume. A hollow or irregular shape needs its own measured net-volume calculation rather than an invented deduction.

Why is the bag count exact while the displayed volume is rounded?＋

The calculation keeps decimal fractions and exact unit factors until the purchasing decision, then formats decimal values for readability. For cylinders, a tightly bounded π approximation must agree on the bag count. Exact arithmetic describes the calculation, not the accuracy of site measurements or actual product yield.

Does extra from bag rounding include my allowance?＋

The allowance is already included in required volume. The separate surplus subtracts that allowance-adjusted requirement from the stated yield of the full-bag purchase. It is an arithmetic surplus and does not guarantee usable leftovers after mixing or placement.

Where can I calculate wall finishes instead?＋

Use Toolvore's Plaster Calculator for its explicit plaster mix and coverage workflow, Paint Coverage Calculator for coats and product coverage, and Tile Calculator for full-box purchasing. Those tasks use different product assumptions from mixed-concrete bag yield.