Elements
Ordering options
How it's calculated
V = L × W × TV = a² × h × nV = (π/4) × d² × h × nV = (π/4) × (D² − d²) × h × n, d < DV = L × [c_d × (c_h + f_t) + g_w × f_t]V = w × run × rise × t(t+1)/2 + landing × w × (R × rise)V_batch = V_net × (1 + waste %)V_order = V_batch rounded up to the supplier incrementV_dry = V_batch × 1.54n is the quantity of identical elements. For stairs, R is the number of risers and t the number of treads below the landing: t = R − 1 with a landing (the top riser steps onto it), t = R without one. Rounding to the supplier's increment applies only to the ready-mix order: truck loads = order volume ÷ truck capacity, rounded up, and cost = order volume × price. Premix bags = batch volume ÷ yield per bag, rounded up. Each site-mix material is its ratio part divided by the sum of the parts, times the dry volume; cement mass = cement volume × 1440 kg/m³. Metric cement bags are 50 kg; US bags are counted as 1 ft³ (94 lb).
Worked example: a porch
A porch slab 4.0 × 3.0 m and 125 mm thick, four round columns Ø300 mm × 2.7 m, and a stair of 4 risers (175 mm rise, 280 mm run, 1.2 m wide) up to a 1.0 m landing. 10% waste, ready-mix rounded to 0.5 m³.
| Element | Working | Volume |
|---|---|---|
| Slab | 4 × 3 × 0.125 | 1.500 m³ |
| Columns | 4 × π/4 × 0.3² × 2.7 | 0.763 m³ |
| Stair treads (3) | 1.2 × 0.28 × 0.175 × (1+2+3) | 0.353 m³ |
| Landing | 1.0 × 1.2 × (4 × 0.175) | 0.840 m³ |
| Net | 3.456 m³ | |
| Batch (+10% waste) | 3.456 × 1.10 | 3.802 m³ |
| Ready-mix order | rounded up to 0.5 m³ | 4.0 m³ (1 truck) |
Mixing it yourself instead: 3.802 ÷ 0.0187 = 203.3, so 204 × 40 kg premix bags. For a 1:2:4 site mix, cement = 3.802 × 1.54 ÷ 7 × 1440 = 1,204 kg (about 24 × 50 kg bags).
Assumptions
- Stairs are modelled as a solid block on grade. A suspended stair with a waist slab needs a different calculation.
- Stair size is entered as the number of risers, the way they are counted on site. Four risers up to a landing means three treads plus the landing.
- Dimensions are as-built concrete sizes with no allowance for formwork tolerances, uneven subgrade or spillage beyond the waste percentage.
- The 1.54 dry-volume factor and 1440 kg/m³ cement bulk density are common rules of thumb. Real values vary.
- Premix bag yields are typical figures. Use the yield printed on your product's bag.
- Site-mix ratios are nominal volume batching (IS 456 convention). UK practice uses BS 8500 standardised mixes (ST1–ST5), proportioned by weight. Mix proportions for structural work must come from a designed mix.
- Reinforcement volume is not deducted. To weigh the steel, use the rebar weight calculator. For the walls above a slab, see the brick and block calculator. For the sub-base under a slab, see the gravel calculator. For digging the footings, see the excavation calculator.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 ft slab, 4 in thick?
The slab is 33.3 ft³ (1.23 yd³). With 10% waste that is 36.7 ft³: 62 × 80 lb bags at 0.60 ft³ each, or 82 × 60 lb bags at 0.45 ft³. Check the yield printed on your brand's bag.
How do I calculate cubic metres (m³) of concrete?
Multiply length × width × thickness, all in metres. A slab 5 m × 4 m × 0.15 m is 3.0 m³, or 3.3 m³ with 10% waste. To convert from cubic yards, 1 yd³ = 0.7646 m³.
How much concrete per square metre?
Volume per m² equals the thickness in metres: a 100 mm slab needs 0.10 m³ per m², and a 150 mm slab 0.15 m³ per m², before waste.
How many 50 kg bags of cement are in 1 m³ of 1:2:4 concrete?
About 6.34 bags: dry volume 1.54 m³, cement share 1/7 = 0.22 m³, at 1440 kg/m³ that is 316.8 kg. This is a nominal volume mix; structural concrete should be a designed mix.
How much extra should I order for waste?
5–10% is typical. Use the upper end for small or awkward pours, uneven ground, or pumping, where some concrete is always left in the lines.
Why does the stair ask for risers, not steps?
Risers are what you count on site. With a landing, the top riser steps onto the landing, so 4 risers means 3 treads plus the landing. Counting it as 4 full steps would overestimate by about 20% in the worked example above.
Standards and references
- BS EN 206 and BS 8500: specifying and ordering concrete (UK/Europe).
- ACI 211.1 (mix proportioning) and ACI 301 (specifications for structural concrete), US.
- Nominal 1:1.5:3, 1:2:4 and 1:3:6 mixes follow the volume-batching convention found in IS 456. Their mapping to modern strength grades is approximate.
This calculator estimates quantities. It does not check strength, durability, cover, loading or any code requirement. Have the design checked by a qualified engineer.