Concrete Calculator

How to measure for concrete

Choose the shape that matches your pour:

• Slab, pad or patio: measure the length and width inside the forms, and decide the thickness in inches or centimetres.
• Strip footing or trench: measure the total length of the footing (add all the runs together), plus its width and depth.
• Round column or post hole: measure the diameter and the height of concrete. For cardboard tube forms with part of the tube above ground, try the Sonotube calculator, which separates depth below and above grade.

Use the quantity field when you have several identical pieces, such as a row of post holes. For anything more complex, such as a slab with a thickened edge, calculate each part separately and add the results.

Concrete volume formulas

The key is to convert every measurement into the same unit before multiplying. In imperial, divide inches by 12 to get feet, multiply to get cubic feet, then divide by 27 for cubic yards. In metric, divide centimetres by 100 to get metres and multiply to get cubic metres directly. The calculator does these conversions for you, so you can enter the length in feet and the thickness in inches.

For bagged concrete, the volume is divided by the yield of one bag. The calculator uses typical premix yields of 0.30 cu ft (40 lb), 0.45 cu ft (60 lb) and 0.60 cu ft (80 lb), and the same yield per kilogram for 20, 25 and 30 kg bags. Check the bag you buy, since yields differ slightly between products.

Worked examples

Slab: a 10 × 10 ft pad, 4 inches thick, with 10% extra.

1. 10 × 10 × (4 ÷ 12) = 33.3 cu ft = 1.23 yd³.
2. Plus 10%: 1.36 yd³, ordered as 1.5 yd³ of ready-mix, or 62 bags of 80 lb.

Footing: 40 ft of footing, 16 inches wide and 8 inches deep, with 10% extra.

1. 40 × (16 ÷ 12) × (8 ÷ 12) = 35.6 cu ft = 1.32 yd³.
2. Plus 10%: 1.45 yd³, ordered as 1.5 yd³, or 66 bags of 80 lb.

Columns: four 12 inch round columns, 4 ft tall, with 10% extra.

1. π × 0.5² × 4 = 3.14 cu ft each, or 12.6 cu ft for four.
2. Plus 10%: 13.8 cu ft (0.51 yd³), or 24 bags of 80 lb.

Metric slab: 3 × 3 m at 10 cm thick is 0.9 m³; with 10% extra, 0.99 m³, or 85 bags of 25 kg.

How much extra concrete?

Adding 5% to 10% is common. Excavations are rarely perfectly flat, forms bow slightly under the weight of wet concrete, and some is always lost to spillage and to the mixer or wheelbarrow. Use the higher end for footings dug directly in soil, since trench sides are irregular, and for pumped pours. Running out of concrete mid-pour can leave a cold joint, so a small surplus is good insurance. Have a plan for leftovers, such as a few small forms for stepping stones.

Ordering ready-mix or bags

The calculator rounds ready-mix up to the nearest quarter yard or quarter cubic metre, since suppliers sell in increments. Ask your supplier about minimum loads and short-load fees, and tell them what you are pouring so they can recommend a mix. For small or scattered pours, bags are often more practical. As a guide, one cubic yard is about 45 bags of 80 lb, which is a lot of mixing; beyond that, a ready-mix truck, a mini-mix trailer or a rented mixer is worth pricing.

To turn volumes into a budget, use the concrete slab cost calculator, which adds ready-mix, bag, labor and extra costs using your own prices.

A note on structure

This calculator estimates quantities only. Footing sizes, slab thickness, reinforcement and depth below frost line determine whether a structure performs safely, and they are governed by local codes. For structural work, follow your approved plans or consult a qualified professional. Wet concrete is caustic, so wear gloves, eye protection and long sleeves, and wash skin promptly if it contacts concrete.

Frequently asked questions

How do I calculate cubic yards of concrete?

Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27. Convert inches to feet by dividing by 12, so a 4 inch slab is 0.333 ft thick. For round columns use π × radius² × height.

How many bags of concrete make a cubic yard?

Using typical bag yields, a cubic yard takes about 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb premix. Always check the yield printed on the bag you buy.

How much extra concrete should I order?

Adding 5% to 10% is common to cover uneven ground, form flex and spillage. Running short partway through a pour is far more costly than a small leftover.

How do I calculate concrete in cubic metres?

Multiply length × width × thickness in metres. Convert centimetres to metres by dividing by 100, so a 3 × 3 m slab that is 10 cm thick needs 0.9 m³ before any extra.

Can I calculate several different shapes at once?

Run the calculator once for each shape, such as the slab and its thickened edge footing, and add the totals. The quantity field handles several identical pieces, like a row of post holes.

Related guides

• Pavers vs Concrete Patio: Cost ComparisonCompare the cost of a paver patio and a poured concrete patio, installed and DIY, with a worked 12 x 16 ft example, upkeep and repair costs, and how to choose.
• How Thick Should a Concrete Patio Slab Be?The standard concrete patio is 4 inches thick on a 4-inch gravel base. When to go to 5 or 6 inches, plus reinforcement, joints and how much concrete to order.

Related calculators

• Concrete Slab Cost CalculatorSlab volume plus ready-mix vs bagged cost, labor and extras at your prices.
• Sonotube Concrete CalculatorConcrete and bags per tube form for deck, porch and post footings.
• Gravel CalculatorVolume and weight for driveways and bases, with presets for common gravel types.
• Paver CalculatorPavers to buy, base gravel, bedding sand and edge restraint for a patio.