Concrete Calculator
Find how much concrete to order for a slab, footing, post hole, stairs or curb, how many bags to buy and what it will cost.
Patios and paths are usually 4 in (10 cm); driveways 5–6 in (12–15 cm).
Leave blank for a solid tube. Fill in for a hollow ring.
Solid stairs sitting on the ground. Leave blank if there is no landing.
For example 12 post holes or 3 matching slabs.
5–10% is usual; more on uneven ground.
Full trucks carry about 8–10 cu yd (6–8 m³). Ask your supplier.
0.45–0.55 is typical. Less water makes stronger concrete.
Pick Slab or Footing to estimate rebar or mesh.
How your concrete is worked out
Volume = length × width × thickness, plus a little extra, then ÷ what one bag makes.
- 1
Sizes in feet
10 ft, 10 ft4 in = 0.333 ft
- 2
Find the volume
10 × 10 × 0.333= 33.33 cu ft
- 3
Add 10% extra
33.33 × 1.1= 36.67 cu ft= 1.36 cu yd
- 4
Count the bags
36.67 ÷ 0.6= 61.1, round up= 62 bags of 80 lb
Read more: the formulas for every shape, a worked example, bag yields and mixing your own
Concrete is sold by volume, so the job is to find the volume of your shape, add a little extra for spills, uneven ground and forms that bow out, and then divide by what one bag makes or what one truck carries.
The formula
slab, footing or wall = length × width (or height) × thickness
column or hole = π × (diameter ÷ 2)² × height
tube = π ÷ 4 × (outside diameter² − inside diameter²) × height
stairs = width × run × rise × (1 + 2 + … + steps) + width × landing × rise × steps
curb and gutter = (curb depth × (curb height + flag thickness) + gutter width × flag thickness) × length
to order = volume × (1 + extra % ÷ 100)
bags = volume to order ÷ what one bag makes, rounded up
cubic yards = cubic feet ÷ 27 · cubic metres = cubic feet × 0.0283
Worked example
A slab 10 ft long, 10 ft wide and 4 in thick. 4 in is 4 ÷ 12 = 0.333 ft, so the volume is 10 × 10 × 0.333 = 33.33 cu ft. Adding 10% extra gives 33.33 × 1.10 = 36.67 cu ft, which is 36.67 ÷ 27 = 1.36 cu yd (1.04 m³). An 80 lb bag makes about 0.60 cu ft, so you need 36.67 ÷ 0.60 = 61.1, rounded up to 62 bags. With 60 lb bags (0.45 cu ft each) it is 36.67 ÷ 0.45 = 81.5, so 82 bags. The slab itself is 0.944 m³ and weighs about 0.944 × 2,400 = 2,265 kg (about 4,990 lb).
What one bag makes
| Bag | Makes about | Bags per cu yd | Bags per m³ |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 118 |
| 50 lb | 0.375 cu ft | 72 | 95 |
| 60 lb | 0.45 cu ft | 60 | 79 |
| 80 lb | 0.60 cu ft | 45 | 59 |
| 20 kg | 9.4 litres | 82 | 107 |
| 25 kg | 11.7 litres | 66 | 86 |
| 40 kg | 18.7 litres | 41 | 54 |
US bag yields are the ones printed on common ready-to-use concrete mixes (80 lb ≈ 0.60 cu ft). Kilogram bags use the same yield per kilogram (about 0.47 litres per kg). Bags per cu yd and per m³ are rounded up. Brands and products differ, so check the bag. Checked 10 October 2026.
Bags or ready-mix?
- Under about 1 cu yd (0.75 m³), bags are usually cheaper and easier. Above that, mixing by hand gets hard work fast: 1 cu yd is 45 bags of 80 lb.
- Ready-mix suppliers often charge a "short load" fee for small orders, so check their minimum.
- Order a little more than you need. Running short in the middle of a pour leaves a weak joint.
Mixing your own: cement, sand and gravel
The dry materials shrink when water is added, as the sand fills the gaps between the stones. So you need about 1.54 times the wet volume in dry materials. That is split by the mix ratio. For 1 m³ of a 1:2:4 mix: dry volume 1.54 m³, 7 parts in all, so cement = 1.54 × 1 ÷ 7 = 0.22 m³ = 0.22 × 1,440 = 316.8 kg (7 bags of 50 kg, as 6.3 rounds up), sand = 0.44 m³ (about 704 kg), gravel = 0.88 m³ (about 1,320 kg) and water = 316.8 × 0.5 = about 158 litres.
The 1.54 factor and the densities used (cement 1,440 kg/m³, sand about 1,600 kg/m³, gravel about 1,500 kg/m³, concrete 2,400 kg/m³) are typical rule-of-thumb figures. Real values change with moisture and the stone size. The grade names next to each mix (M10 to M25) are the usual nominal mixes; structural work should follow an engineer's design.
Rebar and mesh
For a rebar grid, bars run both ways, set in from the edges by the cover. The number of bars in one direction is the inside width ÷ spacing, rounded down, plus one. Add extra for laps where bars join (often 40 bar diameters), and for bent ends. For mesh, sheets = slab area × (1 + overlap %) ÷ sheet area, rounded up.
Common questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in the same unit. A slab 10 ft by 10 ft and 4 in thick is 10 × 10 × 0.333 = 33.33 cu ft. Add 10% extra to get 36.67 cu ft, then divide by 27 for cubic yards: 1.36 cu yd.
How many bags of concrete do I need?
Divide the volume you're ordering by what one bag makes and round up. An 80 lb bag makes about 0.60 cu ft, so 36.67 cu ft needs 62 bags. With 60 lb bags, which make 0.45 cu ft each, it's 82. Brands differ, so check the bag.
How many 80 lb bags of concrete are in a cubic yard?
About 45. An 80 lb bag makes about 0.60 cu ft and a cubic yard is 27 cu ft. A 60 lb bag makes 0.45 cu ft, so it takes 60 of those, and with 40 lb bags it takes 90.
Should I use bags or ready-mix concrete?
Under about 1 cu yd (0.75 m³), bags are usually cheaper and easier. Above that, mixing by hand gets hard work fast, since 1 cu yd is 45 bags of 80 lb. Ready-mix suppliers often charge a short load fee for small orders, so check their minimum.
How much cement, sand and gravel do I need for 1 m³ of concrete?
For a 1:2:4 mix you need about 1.54 m³ of dry materials: roughly 316.8 kg of cement (7 bags of 50 kg), 0.44 m³ of sand and 0.88 m³ of gravel, with about 158 litres of water. These are rule-of-thumb figures; structural work should follow an engineer's design.
How much extra concrete should I order?
Order a little more than you need; the calculator starts at 10% extra. Running short in the middle of a pour leaves a weak joint. The extra you choose is added before bags, trucks and cost are worked out. Change it to suit your job.
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