Concrete Volume Calculator
Note
Calculations assume standard concrete mix. Always order 5-10% extra to account for spillage and variations.
About the Concrete Volume Calculator
Concrete volume is a straightforward geometry problem with an expensive failure mode. Length times width times thickness gives cubic feet; divide by 27 to get cubic yards, the unit ready-mix is sold in. The arithmetic is trivial. What causes problems is the thickness unit — it is almost always given in inches while the other two dimensions are in feet, and mixing them produces an answer twelve times too large.
The asymmetry of errors is what makes this worth doing carefully. Ordering too much wastes perhaps $150 of material. Ordering too little means the pour stops partway, a cold joint forms where fresh concrete meets partially cured concrete, and you have a permanent structural weakness plus a short-load fee and a second delivery charge. Every experienced contractor over-orders deliberately.
Concrete also has no tolerance for delay once it is moving. You cannot pause a pour to run to the supplier — working time is roughly 60 to 90 minutes from batching, less in heat. This is why the standard practice is to add 5% to 10% to the calculated volume before ordering, and to have a plan for the surplus rather than trying to hit the number exactly.
Formula
Cubic yards = (L ft × W ft × T in ÷ 12) ÷ 27 | Cubic metres = L m × W m × T m
- L, W
- Length and width in feet (or metres)
- T
- Thickness, normally quoted in inches — divide by 12 to convert to feet
- ÷ 27
- Cubic feet per cubic yard: 3 × 3 × 3 = 27
- π r² h
- Volume of a cylindrical pier or footing, for round forms
The metric version needs no conversion constant, which is why metric jobs have fewer volume errors. Cubic metres are simply the three dimensions multiplied, with thickness converted from millimetres by dividing by 1,000.
For round footings and sonotube piers, use V = π × r² × h with the radius in feet. A 12-inch diameter tube has a 6-inch radius, which is 0.5 ft — halving the diameter before squaring is a step that gets skipped often enough to be worth calling out.
Irregular shapes should be broken into rectangles and cylinders, calculated separately, and summed. An L-shaped patio is two rectangles; a slab with a thickened edge is a thin rectangle plus a perimeter beam. Trying to average a complex shape into a single rectangle reliably underestimates.
Worked Examples
A 20 ft × 24 ft driveway slab at 4 inches
A standard residential driveway slab, 4 inches thick, poured over compacted base.
Result: Order 7 cubic yards for a calculated 5.93
The 10% allowance covers uneven subgrade, form spread under the weight of wet concrete, and spillage. On a driveway, subgrade variation alone routinely accounts for 5% — a half-inch average dip across this slab is a third of a yard.
Nine sonotube pier footings
A deck on nine 12-inch diameter piers, each 4 feet deep. Round forms, so this is a cylinder calculation.
Result: About 1.15 cubic yards — likely a bagged job rather than a delivery
Small volumes like this fall below most suppliers' minimum, which is typically 1 yd³ with a short-load fee below 3 to 5 yd³. At roughly 45 bags of 80 lb mix this is right at the boundary where renting a mixer beats mixing in a wheelbarrow.
A 10 ft × 12 ft shed slab in bags
A 6-inch thick shed slab where delivery is impractical and bagged mix will be used.
Result: 110 bags of 80 lb mix — about 4.4 US tons of material
This is the calculation that changes the plan. Mixing 110 bags by hand is roughly a full day for two people and physically punishing. Above about 40 bags, ready-mix delivery is almost always cheaper once labour is counted.
How to Use the Result
How much waste to add
The right allowance depends on how much the actual cavity can differ from the drawing:
- —Slab on well-prepared, screeded gravel base — 5% is adequate.
- —Slab on natural or hand-graded subgrade — use 10%, since dips fill with concrete before anything else.
- —Footings and trenches dug by excavator — 10% to 15%, because trench walls slump and machine-dug trenches are rarely exactly to width.
- —Sonotubes and formed piers — 5% is fine; forms are dimensionally accurate.
- —Anything with a thickened edge, haunch or curb — calculate those separately rather than absorbing them into a percentage.
Ordering ready-mix versus buying bags
The crossover is usually somewhere between 1 and 2 cubic yards, and labour is the deciding factor rather than material cost.
Ready-mix suppliers sell in quarter-yard increments with a minimum of about 1 yd³, and apply a short-load fee — commonly $50 to $150 — below roughly 4 to 5 yd³. Delivery also comes with a free unloading window, typically 5 to 7 minutes per cubic yard, after which waiting charges start. Have enough hands ready before the truck arrives.
Bagged mix costs considerably more per cubic yard and requires mixing, but it can be done in stages and needs no truck access. An 80 lb bag yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 50 lb bag about 0.375 ft³ — so it takes 45 of the 80 lb bags to make a single cubic yard.
What thickness the job actually needs
Thickness drives both volume and whether the slab survives. Common minimums:
- —Footpaths and patios — 4 inches, over 4 inches of compacted base.
- —Residential driveways for cars — 4 inches, or 5 inches with reinforcement in freeze-thaw climates.
- —Driveways expecting light trucks or RVs — 6 inches minimum.
- —Garage and workshop floors — 4 to 6 inches depending on loading.
- —Shed and outbuilding slabs — 4 inches, thickened to 8 to 12 inches at the perimeter where walls bear.
- —Frost-depth footings — below the local frost line, which ranges from nothing in mild climates to 48 inches or more in northern regions. This is a code requirement, not a preference.
Costs that do not appear in the volume
The concrete itself is often less than half the total. Budget separately for the base material — typically 4 inches of compacted gravel, which is its own volume calculation — plus reinforcement, forms and finishing.
Reinforcement is priced by area, not volume: welded wire mesh or a rebar grid at 16 to 18 inch spacing for slabs, with vertical bars in piers. Forms are lumber plus stakes, and are usually reusable only once or twice.
Also account for control joints. Cut or tooled at intervals of roughly 24 to 36 times the slab thickness — every 8 to 10 feet on a 4-inch slab — they let inevitable shrinkage cracking happen along a straight line you chose rather than a random one you did not.
Edge Cases and Common Mistakes
Inches and feet in the same calculation
This is the dominant error. Entering 4 as thickness alongside dimensions in feet computes a 4-foot-thick slab — twelve times too much concrete. The result is usually obvious in cubic yards once you know what to expect, so it is worth having a rough benchmark: a 4-inch slab consumes about one cubic yard per 81 square feet.
Sloped and variable-thickness pours
Driveways and patios are usually poured with a fall of about 1/4 inch per foot for drainage, so thickness varies across the slab. Use the average thickness — the mean of the thinnest and thickest points — rather than either extreme. For a slab 4 inches at one edge and 6 at the other, calculate at 5.
Trench footings are wider than the drawing
Machine-dug trenches rarely match their nominal width, and the walls slump before the pour. Measuring the actual excavation rather than trusting the plan dimension is the only reliable approach, and 15% extra is not excessive on hand-dug or clay-soil trenches.
You cannot pause a pour
Concrete begins hydrating immediately and is generally unworkable after 60 to 90 minutes, less above 30 °C. If a pour runs out and a second batch arrives after the first has begun to set, the joint between them is a cold joint — a plane of weakness that will crack and admit water. This is the specific failure that the waste allowance exists to prevent.
Frequently Asked Questions
How many 80 lb bags make a cubic yard?
About 45. An 80 lb bag yields roughly 0.60 cubic feet, and there are 27 cubic feet in a cubic yard. For 60 lb bags it is 60, and for 50 lb bags about 72. Yields vary slightly by manufacturer, so check the bag if the count is tight.
How much extra concrete should I order?
Ten percent is the general default. Drop to 5% for a slab on a properly screeded gravel base where the cavity is well controlled, and go to 15% for machine-dug trenches or anything where the excavation shape is uncertain. Under-ordering costs far more than over-ordering.
What do I do with leftover concrete?
Plan for it before the truck arrives. Pour a spare pad, set a few fence or mailbox posts, cast stepping stones, or form a small equipment pad. Concrete cannot be returned and cannot go down a drain, and most suppliers charge to take it back.
Do I need to include the gravel base in this calculation?
No, calculate it separately. The base sits under the concrete, not within it. A 4-inch compacted gravel base under a 20 × 24 slab is its own 5.93 cubic yards of aggregate, ordered by the ton rather than the yard — roughly 1.4 tons per cubic yard for crushed stone.
How thick does a driveway slab need to be?
Four inches for cars, five to six if trucks, RVs or trailers will use it, and six with reinforcement in freeze-thaw climates. Thickness matters less than base preparation: a 6-inch slab on poorly compacted fill will crack sooner than a 4-inch slab on a properly prepared base.
Can I pour a large slab in sections over several days?
Only if you plan the joints deliberately as construction joints, with keyways or dowels transferring load across them. Accidentally splitting a monolithic pour across two days produces a cold joint that will crack. If the volume is beyond what you can place in one session, design the sections in advance.