Concrete Slab Calculator
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- Runs in your browser
Work out the concrete for a slab, floor, pad or patio: cubic yards, cubic metres and the number of premix bags, with extra for waste.
How it’s calculated
- Volume = length × width × thickness × how many. For a circle, π × diameter² ÷ 4 × thickness.
- 1 cubic yard = 27 cubic feet = 0.7646 m³.
- Bags = volume with extra ÷ what one bag makes, rounded up. Quikrete Concrete Mix makes about 0.30 cu ft (40 lb bag), 0.375 (50 lb), 0.45 (60 lb) and 0.60 cu ft (80 lb). Metric bags are worked out at the same rate: about 0.0094 m³ (20 kg), 0.0117 m³ (25 kg) and 0.0187 m³ (40 kg).
Example: a 10 × 10 ft slab, 4 in thick: 10 × 10 × 4 ÷ 12 = 33.3 cu ft, which is 1.23 cubic yards (0.94 m³). With 10% extra that’s 36.7 cu ft, or 62 bags of 80 lb.
In metric: 3 × 3 m at 10 cm thick is 0.9 m³. With 10% extra, 0.99 m³, or 85 bags of 25 kg.
About Concrete Slab Calculator
For patios, paths, shed bases, garage floors, pads and footings. Enter the size and thickness and you get the concrete in cubic feet, cubic yards and cubic metres, with extra for waste and the number of premix bags in the size you buy. A concrete floor or pad is worked out exactly like a slab, and you can count several pads of the same size at once.
How to use Concrete Slab Calculator
- Choose your units and the shape: a rectangle, or a circle for round pads.
- Enter the length and width, or the diameter, and the thickness.
- If you're pouring several slabs or pads of the same size, enter how many, and pick the extra for waste.
- Choose the premix bag size you buy and read the cubic yards, cubic metres and number of bags.
How your data is handled
Everything is worked out in your browser as you type. Nothing you enter is sent or saved, apart from your choice of metric or imperial units, which is remembered on your device.
Limits and accuracy
- It assumes a flat slab of even thickness. Thickened edges, steps and sloping ground need more; work them out separately and add them.
- Bag yields for 40 to 80 lb bags are Quikrete's figures, and metric bags use the same yield per kilogram. Other brands and mixes differ, so check the bag.
- It doesn't cover the gravel base, reinforcement or formwork. Use the Gravel Calculator for the base.
Questions and answers
How much concrete do I need for a slab?
Multiply length × width × thickness, all in the same unit. A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. In metric, a 3 × 3 m slab 10 cm thick is 0.9 m³.
How many bags of concrete do I need?
Divide the volume by what one bag makes. Quikrete says an 80 lb bag of its Concrete Mix makes about 0.60 cubic feet, a 60 lb bag 0.45 and a 40 lb bag 0.30. The 10 × 10 ft slab above, with 10% extra, is 36.7 cu ft, so 36.7 ÷ 0.6 = 62 bags of 80 lb.
How many cubic yards is a 10 × 10 slab?
At 4 in thick it's about 1.23 cubic yards (0.94 m³), and at 6 in thick about 1.85 cubic yards (1.42 m³), before any extra for waste.
How much extra concrete should I order?
Many people add 5 to 10% for uneven ground, spills and forms that bow slightly, because running short halfway through a pour is worse than having a little left. The calculator adds 10% unless you change it.
Does this work for a concrete pad or floor?
Yes. A pad, a shed base, a patio or a garage floor is a slab: enter its length, width and thickness. For several pads of the same size, such as footings, enter how many.
Should I use bags or ready-mixed concrete?
Bags suit small jobs. A big slab can take dozens of bags and a lot of mixing, so many people order ready-mixed concrete delivered by truck instead, which is sold by the cubic yard or cubic metre.
How thick should a concrete slab be?
It depends on what it carries, the ground under it and local building rules. Patios and paths are often around 4 in (10 cm) and slabs for cars are usually thicker. For a garage, a house or anything structural, follow your plans or ask a professional.