Concrete Calculator
Volume of concrete for a slab, path, or footing — with a conversion to bags of ready-mix so you know exactly what to order.
How this concrete calculator works
Concrete is ordered by volume. The formula is simply:
Volume = length × width × depth
The catch is units: depth is usually measured in millimetres (or inches) while length and width are in metres (or feet), so everything must be converted before multiplying. The calculator handles that, then adds a 10% allowance for uneven ground, spillage, and over-dig — standard practice, because a short pour is a serious problem.
For bagged ready-mix, we convert using the rule that 1 m³ of finished concrete needs roughly 2,200 kg of dry ready-mix (about 108 × 20 kg bags, or 88 × 25 kg bags). In imperial terms, a US 80 lb bag yields about 0.60 cu ft, and a 60 lb bag about 0.45 cu ft.
Common slab depths
| Application | Typical depth |
|---|---|
| Garden path or shed base (light use) | 75–100 mm (3–4″) |
| Patio or driveway (cars) | 100–150 mm (4–6″) |
| Garage floor | 150 mm (6″) |
| Fence post hole backfill | hole depth ≈ ⅓ of post height |
Bags or ready-mixed delivery?
As a rule of thumb: below about 0.5 m³ (⅔ cu yd), mixing bags yourself is practical. Between 0.5 and 1 m³ it's hard work but doable with a mixer. Above 1 m³, get a ready-mix truck or a volumetric ("mix on site") delivery — it's cheaper per cubic metre than bags, far less labour, and the mix quality is consistent. Most suppliers have a minimum load charge, so small deliveries cost proportionally more.
Tips for a successful pour
- Never plan a short pour. Concrete poured against set concrete creates a weak joint. Order 10% extra — the calculator already includes this.
- Check your depth is even. Ground is rarely flat. Measure depth at several points and use the deepest reasonable figure.
- Sub-base matters. A slab is only as good as what's under it: 100 mm of compacted hardcore under most slabs.
- Weather matters. Avoid pouring in frost or heavy rain, and keep concrete damp while curing in hot weather.
Frequently asked questions
How many 25 kg bags of concrete make 1 cubic metre?
Around 88 bags. Dry ready-mix needs roughly 2,200 kg per finished cubic metre: 2200 ÷ 25 = 88. For 20 kg bags it's about 108–110.
How much concrete for a single fence post?
A typical 600 mm deep × 250 mm diameter post hole around a 100 mm post needs roughly 0.025 m³ — about two 25 kg bags of postcrete or ready-mix per post.
What mix ratio should I use if mixing from cement, sand and gravel?
A general-purpose mix is 1 part cement : 2 parts sand : 4 parts gravel (by volume). For foundations, 1:3:6 is common; for hard-wearing slabs, 1:1.5:3.
How long before I can walk or drive on new concrete?
Foot traffic after 24–48 hours, light vehicles after about 7 days, and full strength takes around 28 days.
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