Concrete calculator: how much concrete do I need?

How to use the concrete calculator

Pick a shape, enter its dimensions and how many there are, add any more items, and set a wastage allowance. The result is the volume to order, in the units you use.

1. Choose Region presets and Units at the top. They set the bag sizes and the units you type in; you can change either at any time.
2. For a slab, footing, wall or beam choose Slab / footing / wall and enter length, width and thickness. Thickness is in mm or inches.
3. For round piers and sonotubes choose Round column; for pipe sleeves or hollow piers, Tube; for a flight of solid steps on the ground, Steps.
4. Use Quantity for identical items (eight footings, twelve posts) and Add another item for different ones.
5. Open Mixing on site to change the mix ratio, cement bag size or dry-volume factor, and Cost to enter your local prices.

Concrete volume formulas by shape

Concrete volume is the space the shape encloses: length × width × thickness for a slab, π × radius² × height for a round column. Work in one unit, then convert.

Slab, footing, wall, beam   V = L × W × T
Round column                V = π × (D ÷ 2)² × H
Tube                        V = π ÷ 4 × (D² − d²) × H
Steps on the ground         V = R × G × n(n + 1) ÷ 2 × W  (+ top platform)

Convert before you multiply: 4 inches is 0.333 ft and 100 mm is 0.1 m. Then 1 m³ = 35.31 ft³ and 1 yd³ = 27 ft³ = 0.7646 m³ (exact definitions of the foot and yard, NIST). In Pakistan and India sand and aggregate are usually priced per cubic foot (cft), so the calculator shows ft³ too.

Worked example: a 3 × 3 m slab, 100 mm thick

3 m × 3 m × 0.1 m = 0.9 m³. With 5% wastage, order 0.95 m³ (1.24 yd³). Mixed on site at 1:2:4 that is 6 bags of 50 kg cement, 0.42 m³ of sand and 0.83 m³ of aggregate.

1. Volume: 3 × 3 × 0.1 = 0.9 m³.
2. Add 5% wastage: 0.9 × 1.05 = 0.95 m³.
3. Dry materials: 0.95 × 1.54 = 1.46 m³.
4. Cement (1 part in 7): 1.46 ÷ 7 = 0.21 m³ × 1,440 kg/m³ = 299.4 kg = 5.99 bags, so buy 6.
5. Sand (2 parts): 0.42 m³ (14.7 cft). Aggregate (4 parts): 0.83 m³ (29.4 cft).

How much concrete for a 10 × 10 slab and other common sizes

Ready-mix suppliers sell by the cubic yard or m³, often in steps of ¼ or ½; round up to their step.

How many bags of premix concrete do I need?

Divide the volume in cubic feet by the bag's yield. An 80 lb bag of standard concrete mix yields about 0.60 ft³, so one cubic yard (27 ft³) takes 45 bags.

Yields are from the QUIKRETE Concrete Mix data sheet; other brands' standard mixes are similar, but lightweight or high-yield mixes differ, so check the bag. Bagged premix makes sense for posts and small pads; past about one cubic yard, ready-mix is usually cheaper and more consistent.

Cement, sand and aggregate for site-mixed concrete

Multiply the wet volume by a dry-volume factor of about 1.54, split it by the mix ratio, and convert the cement share to kilograms at 1,440 kg/m³. For 1 m³ of 1:2:4 that is 6.34 bags of 50 kg cement, 0.44 m³ of sand and 0.88 m³ of aggregate.

Dry cement, sand and stone take up more room loose than they do once mixed with water and compacted, because the voids fill. The 1.54 factor (often quoted as 1.52–1.57) is a site convention for nominal mixes, not a value from a standard, so the calculator lets you change it.

Cement's loose bulk density of 1,440 kg/m³ is from IS 875 Part 1, so a 50 kg bag holds 0.0347 m³ (1.23 cft). The grade labels follow site practice for IS 456 nominal mixes, which the code allows only up to M20; IS 456 itself batches nominal mixes by mass, which gives about 10% less cement for M20 than this volume method. For anything structural, use the mix your engineer specifies. For other mixes and for mortar and plaster, use the cement, sand and aggregate calculator.

How much extra concrete to order

Add 5–10% to the calculated volume. The extra covers uneven sub-grade, formwork that bows, spillage and what stays in the mixer or pump.

The National Ready Mixed Concrete Association puts the usual allowance at 4–10% (CIP 31), which is why the calculator starts at 5%. Use the top of the range for slabs on rough ground or footings dug by hand, where the trench is rarely exactly the drawn size.

Assumptions and limits

The calculator measures geometry and converts it to materials. It doesn't design concrete: it doesn't choose a grade, check strength or size reinforcement.

• Volumes assume the dimensions you enter are the poured size. Thickened slab edges, sloping ground and over-dig need their own items or a bigger wastage figure.
• Steps are solid steps on the ground. A suspended (waist-slab) staircase uses less concrete; measure the waist slab as a slab plus the triangular steps.
• Site-mix quantities are for nominal volume batching with dry sand. Damp sand bulks, taking up more room for the same weight, so measure it dry or allow for the difference.
• Costs appear only for prices you enter. The calculator never guesses local prices.

Pricing a whole house rather than one pour? Use the construction cost estimator instead.

Sources

Constants and limits come from these standards, manufacturers' data and trade bodies. Where practice varies, the calculator lets you change the value.

• QUIKRETE Concrete Mix (No. 1101) data sheet: bag yieldsThe QUIKRETE Companies · October 2022
• IS 875 (Part 1):1987 Unit weights of building materials (Table 1)Bureau of Indian Standards · 1987
• IS 456:2000 Plain and Reinforced Concrete: Code of Practice (Table 9, nominal mixes)Bureau of Indian Standards · 2000, reaffirmed
• CIP 31: Ordering ready mixed concrete (waste allowance)National Ready Mixed Concrete Association · 2021
• NIST Guide to the SI, Appendix B.9: conversion factorsNational Institute of Standards and Technology · SP 811

Found a value that doesn’t match your local practice? Tell us and we’ll review it.