Concrete Calculator
How much concrete do you need? Slab, footing, round column or steps — volume in yd³ or m³, bag counts for 40/60/80 lb and 25 kg mixes, waste and cost.
Bag counts — with waste, per size
What this calculator does
Tell it what you’re pouring — a slab, a strip footing, a round column in a tube form, or a flight of steps — and it returns the volume in cubic yards (or m³), the pour weight, and the bag count at real manufacturer yields, with the waste margin you choose. Prices are optional: type a bag price and a ready-mix rate and it shows the two totals side by side, because past a yard or so the truck usually wins on money, not just effort. Everything recomputes as you type, in feet+inches or metres+centimetres, and the URL rewrites itself so you can share or bookmark a calculation.
The math, in the open
slab / footing = L × W × D · column = π × (d ÷ 2)² × h · yd³ = ft³ ÷ 27
steps ≈ W × T × R × n(n+1) ÷ 2, plus platform W × P × (n × R) · bags = ceil(volume with waste ÷ bag yield)
- Bag yields are published figures: 80 lb → 0.60 ft³, 60 lb → 0.45, 40 lb → 0.30 per QUIKRETE’s data sheet; 25 kg → 0.0116 m³ per SAKRETE/Basalite’s. Every number the tool asserts is listed on the sources page.
- Steps are an honest approximation: the stair is modeled as n stacked boxes of W × T × (i × R) — quick and close, but real pours shift with riser forms and nosings.
- Weight is a typical figure: ~150 lb/ft³ (2,400 kg/m³) for normal-weight concrete — right for floor-load sanity checks, wrong enough that engineers specify mixes properly.
Worked example — a 10 × 10 ft shed slab, 4 in thick
33.3 ft³ exact = 1.23 yd³. With the default 10% margin that’s 36.7 ft³ → 1.36 yd³ to order, 62 × 80 lb bags (or 82 × 60 lb, 123 × 40 lb), weighing about 5,500 lb as poured. At $6 per 80 lb bag that’s ~$370 in bags — worth comparing against the ready-mix line, where 1.4 yd³ often prices lower even with a short-load fee.
The other materials
Same honest math for the rest of the job: gravel and crushed stone, mulch, topsoil, and a paver calculator that counts the bricks and the base and bedding under them. Guides: how much concrete you actually need, how deep mulch should be, and gravel types, sizes and weights. Sister site PaintCalc does the same job for paint.
Gravel Calculator
Square footage to cubic yards and tons — density by material, editable.
Open →Mulch Calculator
Beds × depth to cubic yards — and whether bags or bulk wins.
Open →Topsoil Calculator
Cubic yards and tons for lawns, beds and level-outs.
Open →Paver Calculator
Brick count plus the gravel base and bedding sand under it.
Open →Frequently asked questions
How do I calculate how much concrete I need for a slab?
Length × width × thickness, all in feet: a 10 × 10 ft slab at 4 in thick is 10 × 10 × (4 ÷ 12) = 33.3 ft³, and dividing by 27 gives 1.23 yd³. Add ~10% for spillage and an uneven subgrade — that's ~1.36 yd³, or 62 × 80 lb bags. The calculator above does this as you type, and the thickness guide covers what depth your project needs.
How many bags of concrete do I need for a 10 × 10 slab?
At 4 in thick the slab needs 33.3 ft³, or 36.7 ft³ with a 10% margin. At the manufacturer's published yields that's 62 × 80 lb bags (0.60 ft³ each), 82 × 60 lb (0.45 ft³) or 123 × 40 lb (0.30 ft³). Mixing 60+ bags by hand is a real day's work — past roughly half a yard, price out a ready-mix truck too.
What's the difference between cubic feet, cubic yards and bags?
Bulk concrete is sold by the cubic yard (27 ft³); bagged mix is sold by weight — 40/60/80 lb bags yielding 0.30/0.45/0.60 ft³. So one yard of concrete is exactly 45 × 80 lb bags of volume (27 ÷ 0.60), or 60 × 60 lb bags. Metric: a 25 kg bag yields 0.0116 m³, so ~86 bags per m³.
How much does the concrete itself weigh?
Normal-weight concrete runs about 150 lb per ft³ (≈4,050 lb/yd³, 2,400 kg/m³) wet or cured — the card above multiplies your pour out so you can sanity-check floor loads or trailer capacity. It's a typical figure, not a spec: lightweight and dense mixes exist.
Should I order 5%, 10% or 15% extra?
10% is the standard margin for slabs and footings over a compacted, level base — it covers spillage, slight over-excavation and form wobble. Bump to 12–15% if the ground is soft or irregular, or the formwork is rough. Running short mid-pour is the expensive outcome: a second truck or an unplanned cold joint.
Does the calculator work in metric?
Yes — switch the units and every field converts exactly (10 ft ↔ 3.05 m, 4 in ↔ 10.2 cm), volumes read in m³, bags become 25 kg sacks at 0.0116 m³ each and prices shift to per-m³. Your typed numbers round-trip back exactly when you switch units again.
Can I share a calculation?
The URL rewrites itself as you type — copy the address bar and it reopens with the same shape, dimensions, units, waste and prices. Nothing is stored or sent anywhere; the whole calculator runs in this browser tab.