How We Calculate
Every calculator here follows the same few rules: add everything up first, round once at the unit you buy, and show where each figure comes from.
One rounding rule
A calculator can round at every step or only at the end. We round only at the end. Each part of a job is worked out exactly, the parts are added into one total, any waste allowance is put on that total, and the result is rounded up once, to the unit you actually buy: a whole bag, a whole sheet, a whole roll.
Rounding each part first can add bags that the total never needed. The table below shows three jobs. An exact fit is not rounded up. A job made of several parts is bought from the total, not from each part's own count.
Worked examples of rounding once
Bags of the maker's concrete mix in the 80 lb bags size, by job, with no waste added
| Job | Exact bags | Bags to buy, rounded once | Bags if each part were rounded |
|---|---|---|---|
| Slab, 24 ft × 24 ft × 6 in | 480 | 480 × 80 lb bags | 480 × 80 lb bags |
| Slab, 9 ft × 9 ft × 4 in | 45 | 45 × 80 lb bags | 45 × 80 lb bags |
| Slab, thickened edge all round it and piers together | 104.15 | 105 × 80 lb bags | 108 × 80 lb bags |
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
The first column of counts is the exact amount. The last is what you would buy if every part of the job were rounded up on its own and then added.
The yield used in these examples
What one bag of the maker's concrete mix yields, by bag size
| Bag size | Yield per bag |
|---|---|
| 40 lb | approx. 0.3 ft³ |
| 50 lb | approx. 0.375 ft³ |
| 60 lb | approx. 0.45 ft³ |
| 80 lb | approx. 0.6 ft³ |
| 90 lb | approx. 0.675 ft³ |
Figures in the Yield per bag column are per QUIKRETE's data sheet, checked Nov 12, 2026.
Product names are trademarks of their owners. HoneyDoMath is not affiliated with, sponsored by or endorsed by these makers. Figures come from each maker's published sheets and are shown for information only.
Source: [1]
Your bag's label comes first. If it says something different, use the label.
Imperial and metric
Pages show US (imperial) units. A switch on each calculator shows the same figures in metric. A figure from a source stays in the unit its source used.
Where the numbers come from
A number is shown as a fact only when it has a direct source, cited on the same page. We use three kinds of source, each only for its own kind of fact. Model codes and government or university extension sources are used for code limits and guidance. A maker's own data sheet or label is used only for that maker's product: its yield, coverage, size or count. Standards and industry bodies are used for estimating practice. We do not use blogs, retailer pages or forums.
A maker's figure is always shown as that maker's, with the date we checked the sheet. Using a maker's figure does not mean the maker is connected to this site.
Code text is quoted and compared in neutral words: within or outside the model-code value. We quote only a short excerpt, never more than fifty words. Local codes and amendments differ, and your building department decides.
Where sources disagree, we show a default, name the other value and let you change it. A value we cannot confirm from a source is not shown as a fact. It is an input you set, with a note to check your label or supplier. Each source shows the date we checked it.
Constants and tests by family
Each family of calculators has a set of test jobs, and its results are tested against worked examples from the cited sources. A family's test jobs appear here once its calculators are on the site.
Concrete: test cases
Bags to buy for each test job in the 80 lb bags size, with no waste added
| Job | Bags to buy |
|---|---|
| Slab, 10 ft × 10 ft × 4 in | 56 × 80 lb bags |
| Slab, 24 ft × 24 ft × 6 in | 480 × 80 lb bags |
| Slab, 12 ft 6 in × 9 ft 3 in × 4 in | 65 × 80 lb bags |
| Slab, 9 ft 10 1/8 in × 13 ft 1 1/2 in × 3.937 in | 71 × 80 lb bags |
| Round column, 12 in across, 4 ft deep | 6 × 80 lb bags |
| Round slab, 10 ft across, 4 in thick | 44 × 80 lb bags |
| Slab, thickened edge all round it and piers together | 105 × 80 lb bags |
Each result is tested against a worked example from the cited sources.
Estimates, and your data
Results are estimates for planning, worked out from the sizes you enter and the published product data. They are not professional advice. Check your product's label, your supplier and your local rules before you buy or build.
Calculations run in your browser. What you enter stays on your device.
Calculators that use these rules
Sources (1)
-
QUIKRETE Concrete Mix No. 1101 data sheet (opens in new tab), The Quikrete Companies, Revised October 2022.
Manufacturer data.
“A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L)”
“A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L)”
“A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L)”
“An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L)”
“A 90 lb (40.8 kg) bag yields approximately 0.675 ft3 (19.1 L)”
Per QUIKRETE's data sheet, checked Nov 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1)
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.