Concrete Bag Calculator

Guide: the yield figures this tool depends on — where the manufacturer states them, why the pound sizes scale in a straight line while the kilogram sizes do not, and how to read the bag in your hand — are explained in Concrete bag yields by size. This page documents what the calculator does with those figures.

How a volume becomes a bag count

For each size, bags = ceil(volume × (1 + waste%) ÷ yield per bag). The worked 10 cu ft project grows to 11 cu ft with waste, and the engine-computed counts are:

• 40 lb bags (0.3 cu ft each): 37 bags, 0.1 cu ft spare
• 50 lb bags (0.375 cu ft each): 30 bags, 0.25 cu ft spare
• 60 lb bags (0.45 cu ft each): 25 bags, 0.25 cu ft spare
• 80 lb bags (0.6 cu ft each): 19 bags, 0.4 cu ft spare

Waste is applied to the volume before the round-up to whole bags. Doing it the other way — padding the finished bag count — double-charges the margin on small jobs.

That one formula is the site's five-step estimation pipeline with the first step already done. Geometry happens elsewhere: this tool takes a finished volume, either because you measured one directly or because a sibling tool computed it from dimensions. Coverage divides that volume by the placed yield of one bag. Waste multiplies the volume by (1 + waste% ÷ 100) first — the order matters, for the reason the previous paragraph gives. Package rounding takes the ceiling, because a store sells whole bags and never four-fifths of one. User-priced cost, if you typed a price, multiplies the rounded count for the one size you said you would buy. Intermediate figures keep full precision; only the displayed values are rounded, and the bag count is never rounded to the nearest — always up.

Running that pipeline once per bag size, side by side, is the point of this tool. Each row is an independent answer to "how many if I bought only this size?", so the rows are alternatives to choose between, not a shopping list to add together.

What each input does

Volume and unit. Imperial mode takes cubic feet or cubic yards; a cubic-yard entry is multiplied by 27 before it reaches the engine, because a yard is three feet and a cubic yard is that cubed — a definition, not a measurement. Metric mode takes cubic metres or litres and converts them exactly: one cubic foot is 0.3048 m cubed, about 0.028317 m³, and one cubic metre is 1,000 L. Whatever you type, the engine sees cubic feet, so the imperial and metric arithmetic is identical; only the bag table and the displayed units change.

Waste allowance. The default of 10% is a trade convention for concrete pours — spillage and an uneven subgrade — not a manufacturer figure, which is why the field is editable. The engine accepts any percentage from zero to one hundred and rejects anything outside that range. Enter zero to see the pure geometry-divided-by-yield count; the waste factors guide covers how to choose a figure for your own conditions.

Bag size you plan to buy. This selector changes nothing in the count rows — they always show every size. It only tells the cost step which row to multiply by your price.

Price per bag. Optional, and supplied only by you: the tool has no price table, and with the field empty no cost line appears. Enter a price and the cost is that price times the rounded bag count for the selected size. It is the rounded count deliberately — a bag you must buy but will not fully use is paid for in the estimate, as it would be at the register.

Yields, per the data sheet

The four pound sizes come from the QUIKRETE Concrete Mix (Product No. 1101) data sheet, which states the placed volume of each bag as approximate. They are manufacturer figures, recorded in the site's materials data with their source and verification date and pinned by an automated test, so a published change is a deliberate edit rather than a drift. Every size of that mix places 0.0075 cu ft per pound of dry mix: 40 lb → 0.3 cu ft, 50 lb → 0.375 cu ft, 60 lb → 0.45 cu ft, 80 lb → 0.6 cu ft. That regularity is why doubling the bag weight exactly halves the count before rounding — and why, after rounding, the counts halve exactly only when both divisions land on whole bags; otherwise the two ceilings drift apart.

The two kilogram sizes come from a different sheet — Target Products, the licensed manufacturer of the same mix in western Canada — and they are manufacturer figures in their own right, not the pound figures converted: 25 kg → 0.43 cu ft (12.2 L), 30 kg → 0.52 cu ft (14.7 L). The distinction matters. A 30 kg bag weighs 66.1 lb, which at 0.0075 cu ft per pound would imply 0.496 cu ft — but the manufacturer publishes 0.52 cu ft. Pre-blended mixes are batched and rated per bag, so the tool uses the published metric yields directly and never scales the pound table.

The same data sheet lists a larger regional bag that this tool does not offer, and other brands publish their own yields, which are close to these but not identical. If your bag states a different placed volume, the counts here are a starting point: divide your waste-adjusted volume by the figure printed on your bag and round up, which is the same arithmetic as the formula above.

Worked examples

Every figure below is produced at build time by the same engine the form runs, with the default 10% waste unless the example says otherwise. Read them as a tour of how the pipeline behaves at different volumes, not as templates for a particular job.

One cubic yard

The question in the unit US suppliers sell concrete in. One cubic yard is 27 cu ft; with 10% waste the engine mixes 29.7 cu ft, and the four rows read 50 × 80 lb (0.3 cu ft spare), 66 × 60 lb (0 spare), 80 × 50 lb (0.3 spare), or 99 × 40 lb (0 spare). At zero waste the same yard needs 45, 60, 72, and 90 bags — the no-waste column of the table above — and because 27 happens to divide evenly by all four yields, each of those counts leaves 0 cu ft spare.

The 10 × 10 ft slab at 4 in

The methodology page's slab: 100 sq ft × ⅓ ft = 33.33 cu ft (1.23 cu yd), taken here from the concrete slab engine and handed to this one. With waste it becomes 36.67 cu ft, which needs 62 × 80 lb bags with 0.53 cu ft spare, or 82 × 60 lb, 98 × 50 lb, or 123 × 40 lb. This is exactly the cubic-feet figure the concrete slab calculator reports, and carrying it here is the intended workflow when a pour is being considered in bags rather than by the truck.

A small pour: 2.5 cu ft

Waste-adjusted, 2.5 cu ft becomes 2.75 cu ft. The rows read 5 × 80 lb (0.25 cu ft spare), 7 × 60 lb (0.4 spare), 8 × 50 lb (0.25 spare), and 10 × 40 lb (0.25 spare). On a job this small the round-up dominates: the 60 lb row leaves the most unused concrete, 0.4 cu ft, which is 14.5% of the volume being mixed. The smallest bag is not automatically the closest fit — where the volume lands relative to each size's whole-bag multiples decides, and the spare column is the only place that shows.

An exact fit: 6 cu ft at zero waste

6 cu ft divides evenly by 3 of the 4 yields: exactly 20 × 40 lb, 16 × 50 lb, 10 × 80 lb, each with 0 cu ft spare. The 60 lb row does not divide evenly — 13.33 bags — and rounds up to 14, leaving 0.3 cu ft. The engine treats a quantity within a vanishingly small relative tolerance of a whole bag as a whole bag, so binary floating-point noise on an exact division never buys a phantom bag; the rounding you see is always a real remainder.

The same 10 cu ft at 0%, 10%, and 15% waste

Waste moves the mixed volume in a straight line — 10, 11, and 11.5 cu ft — while the count moves in whole-bag steps: 17, 19, and 20 × 80 lb, or 34, 37, and 39 × 40 lb. The steps land unevenly — on the 80 lb row, 2 bags and then 1 — because the count only moves when the adjusted volume crosses a whole-bag boundary, and a few points of waste can just as well add nothing: at 12% the mix is 11.2 cu ft and the 80 lb count is still 19, with 0.2 cu ft spare, while the 40 lb count has already stepped from 37 to 38. The spare figure tells you how close each count sits to its next step.

Metric: 0.5 m³

Half a cubic metre is 17.66 cu ft to the engine. With 10% waste it mixes 19.42 cu ft (550 L), and the metric table reports 38 × 30 kg bags with 9.5 L spare, or 46 × 25 kg with 10.1 L spare. The litres shown are the engine's cubic-feet figures converted back for display; nothing about the count depends on which unit you typed.

Metric: 150 L

A volume small enough to be thought of in litres — 150 L is 5.3 cu ft — mixes to 5.83 cu ft (165 L) with waste and needs 12 × 30 kg bags (11.7 L spare) or 14 × 25 kg (5.5 L spare). The litre unit is converted through cubic metres (1,000 L each) and then the exact cubic-metre-to-cubic-foot factor, so no rounding enters before the bag division.

Two cubic yards

2 cu yd is 54 cu ft, 59.4 cu ft with waste, and the counts run to 99 × 80 lb, 132 × 60 lb, 159 × 50 lb, or 198 × 40 lb. The arithmetic is no different at this scale; what changes is the comparison worth making. The concrete slab calculator reports the same volume rounded up to the next 0.25 cu yd — a common ready-mix ordering increment that the slab tool treats as a trade convention to confirm with the supplier, not a specification — and whether bags or a truck suits a pour of this size is a decision this tool leaves to you.

Reading the output

The first line is the waste-adjusted volume — what you will actually mix, in cubic feet or, in metric mode, cubic metres and litres. Below it is one row per bag size: the whole-bag count and the spare figure, which is the concrete those bags provide beyond the waste-adjusted volume. Spare sits on top of the waste allowance, not inside it. A cost line appears only when you have entered a price, and only for the size you selected. The copy button captures the mixed volume and every size's count in one line, ready to paste into a shopping list.

Common misreadings

Cubic yards typed into the cubic-feet unit. One yard entered as "1" with the unit left on cubic feet hands the engine one cubic foot where 27 were meant — the volume is short by a factor of 27, and the bag count follows it; the reverse mistake over-supplies the volume by the same factor. The unit selector exists so the conversion is never done in your head.

Slab dimensions multiplied without converting inches. The house example: 10 ft × 10 ft × 4 in is not 400 cu ft, it is 33.33 cu ft, because the 4 inches must become ⅓ ft before the multiplication. Type 400 and the engine, doing its job faithfully, asks for 734 × 80 lb bags instead of 62 — 11.8 times the order, on 12 times the volume; the two ratios differ because counts round up to whole bags and do not scale exactly. If a bag count looks absurd, check whether the volume was built from a thickness in inches.

Adding the rows together. The four counts are four ways of buying the same volume. Summing them buys the job four times over.

Applying waste twice. The waste allowance is already inside every count. Adding "a couple of extra bags for safety" on top is a second margin — reasonable if that is what you want, but know that it is a second one.

Reading spare as waste. Spare is the rounding remainder above the waste-adjusted volume, not the waste itself. In the one-cubic-yard example the 60 lb row shows 0 cu ft spare and still carries the 10% waste that took the volume from 27 to 29.7 cu ft.

Scaling the pound table for kilogram bags. As the yield section shows, the manufacturer's published metric yields differ from a weight-proportional estimate. Use metric mode, which carries the published figures, rather than converting a kilogram bag to pounds and reading the pound row.

Where the model stops

This is a purchasing count, and the site estimates material quantities only. It does not determine whether a mix, a thickness, or a placement suits the job structurally, whether it complies with a building code, or how the ground beneath it will behave — those are questions for a licensed professional, exactly as the note under the form says.

Within its own job, the model simplifies. It assumes the volume you enter is the volume that gets placed; it does not model formwork, subgrade preparation, reinforcement, moisture and temperature effects, regional product variations, or delivery minimums. The manufacturer states its yields as approximate, and the tool carries them as exact. Waste is one flat percentage applied to the whole volume. Bags are whole: the tool never plans a part bag or a combination of sizes. Water, mixing equipment, and the labour of mixing are outside the model entirely, and so is any price the tool did not receive from you.

Tools that feed this one

Not sure of the volume yet? The concrete slab calculator, concrete footing calculator, concrete column calculator, and post hole calculator each compute their own geometry and report bag counts with these same verified yields.

For a shape none of those cover, the volume calculator turns dimensions into cubic feet, and the construction unit converter moves any figure between cubic feet, cubic yards, cubic metres, and litres. The concept pages behind this tool are Concrete bag yields by size, Cubic feet, cubic yards, and ordering concrete, and Waste factors, explained; the full list is on the guides index.

Frequently asked questions

Everything computes locally in your browser; no volume you enter is stored or sent. Sources for the yield constants are cited on the methodology page and in the engine itself.