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Concrete Calculator
Add each slab, footing, post hole, wall or set of steps to get the concrete in cubic yards, cubic feet and cubic metres — and how many premixed bags or how much ready-mix to order.
An estimate for ordering. Ready-mix suppliers set their own minimum loads, order increments and short-load or delivery fees — ask yours before you book. A cubic yard is 0.7646 m³.
How it's calculated
- Slab, wall or strip footinglength × width (or height) × thickness
- Square footing or columnside × side × depth × quantity
- Round hole or columnπ × (diameter ÷ 2)² × depth × quantity
- Stepswidth × run × rise × n(n + 1) ÷ 2 + width × platform × n × rise
- To ordertotal × (1 + extra %) · bags = ⌈volume ÷ bag yield⌉
1 ft = 0.3048 m and 1 in = 2.54 cm exactly; 1 yd³ = 27 ft³ = 0.764555 m³. Bag yields: QUIKRETE Concrete Mix No. 1101 (80 lb 0.60 ft³, 60 lb 0.45 ft³, 40 lb 0.30 ft³) and QUIKRETE 30 kg Concrete Mix (0.014 m³); 20 and 25 kg are pro-rated from the 30 kg yield. Checked 2026-10-08.
Worked examples
A 10 × 10 ft patio, 4 in thick: 10 × 10 × (4 ÷ 12) = 33.33 ft³. With 10% extra that's 36.67 ft³ = 1.36 yd³, or 36.67 ÷ 0.60 → 62 × 80 lb bags (82 × 60 lb).
One post hole 10 in across and 36 in deep: π × (5 ÷ 12)² × 3 = 1.64 ft³ — 3 × 80 lb bags or 4 × 60 lb before any extra (the post itself takes up some room).
A 4 × 3 m slab, 10 cm thick: 1.2 m³, or 1.32 m³ with 10% extra — about 95 × 30 kg or 114 × 25 kg bags, a job where ready-mix is usually easier.
Frequently asked questions
How do I calculate how much concrete I need?+
Multiply length × width × thickness in the same unit to get the volume, then add 5–10% for spillage and uneven ground. A 10 × 10 ft slab 4 in (1/3 ft) thick is 33.33 ft³, or 1.23 cubic yards; with 10% extra it's 36.67 ft³ = 1.36 yd³.
How many bags of concrete do I need?+
Divide the volume by the yield printed on the bag and round up. QUIKRETE lists 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb. For 36.67 ft³ that's 62 × 80 lb, 82 × 60 lb or 123 × 40 lb bags.
How many 80 lb bags of concrete are in a cubic yard?+
At 0.60 ft³ per bag, 27 ÷ 0.60 = 45 bags. At 0.45 ft³, a cubic yard takes 60 × 60 lb bags; at 0.30 ft³, 90 × 40 lb bags. That's before any extra for waste.
How many 25 kg bags of concrete make a cubic metre?+
It depends on the mix. QUIKRETE's 30 kg Concrete Mix yields 0.014 m³, so a cubic metre takes about 72 × 30 kg bags. Pro-rated, that is about 86 × 25 kg or 108 × 20 kg bags. Check the yield printed on the bag you buy.
Should I use bags or ready-mix?+
Bags suit small jobs such as post holes, pads and small slabs. Above roughly a cubic yard or a cubic metre, mixing by hand gets slow and ready-mix is often easier. Ready-mix suppliers set their own minimum loads and order increments, often quarter or half yards, and many add a short-load or delivery fee for small orders. Ask your supplier for their minimum and fees before you compare.
How are concrete steps calculated?+
Each step is treated as a solid block from the ground up, so step 1 is one rise high, step 2 two rises and so on. Volume = width × run × rise × n(n + 1) ÷ 2, plus any top platform at the full height. Three steps 1 m wide with an 18 cm rise and 28 cm run take 0.302 m³.
Is anything I enter saved or sent?+
No. Everything is calculated in your browser and nothing is uploaded or stored.