How many cubic metres of concrete do I need for a slab, footing or post holes?

How does the concrete calculator work?

It converts millimetres to metres, works out the volume, and adds 10%, exactly as MeisterCalc's Concrete calculator does.

• Slab or strip footing: volume (m³) = length (m) × width (m) × depth (m). Divide millimetres by 1000 first: 100 mm is 0.1 m.
• Post holes: volume per hole = π × radius² × depth, with the radius half the diameter, in metres. Multiply by the number of holes.
• Litres: 1 m³ = 1000 litres.
• Order: volume × 1.10. The 10% covers an uneven base, low spots, overpour along the formwork and what's left in the chute or the barrow.
• Bags: order volume × the maker's bags per m³, rounded up to a whole bag.

Worked example: a 4000 × 3000 mm slab, 100 mm thick

1. Convert to metres: 4 m × 3 m × 0.1 m.
2. Multiply: 4 × 3 × 0.1 = 1.2 m³, which is 1200 litres.
3. Add 10%: 1.2 × 1.10 = 1.32 m³ to order.

In 20 kg bags of Boral Concrete Mix (108 bags per m³ on its data sheet), 1.32 m³ is 143 bags. That's a lot to mix by hand, so for a slab this size get a ready-mix price too. The sum is 1.32 × 108 = 142.6, rounded up.

Worked example: six post holes, 300 mm across and 800 mm deep

1. Radius = 300 ÷ 2 = 150 mm = 0.15 m.
2. Area of the hole = π × 0.15² = 0.0707 m².
3. Per hole = 0.0707 × 0.8 = 0.0565 m³ (56.5 litres).
4. Six holes = 0.3393 m³ (339.3 litres).
5. Add 10%: 0.3732 m³ to order.

That's 38 bags of 25 kg Dricon HandiCrete (0.010 m³ each, Dricon data sheet) or 41 bags of 20 kg Boral Concrete Mix (108 per m³, Boral data sheet). The post itself takes up some of the hole: a 100 × 100 mm post 800 mm deep fills 0.008 m³, so six of them save about 0.048 m³. The calculator doesn't take that off, so you'll have a little over.

Worked example: a strip footing

A strip footing is a long box, so it uses the slab formula. For 20 m of footing, 300 mm wide and 400 mm deep: 20 × 0.3 × 0.4 = 2.4 m³, and 2.64 m³ to order.

Measure the length along the centre of the trench. Around a rectangle, the centre-line length gives the right volume at the corners; the outside length counts each corner twice.

How many bags make a cubic metre?

It depends on the product, so the calculator only counts bags for products whose maker publishes a yield. These are the figures on each maker's own data sheet:

Checked on 28 October 2026. Bags per m³ is 1 ÷ the stated yield where the maker gives m³ per bag. Every figure is the maker's approximation for its own product mixed as its instructions say; another brand, or the same brand's rapid-set or high-strength mix, can differ. Read the bag before you buy. Cement Australia's standard Concrete Mix, 20 kg, isn't in the calculator because its sheet gives coverage, not a yield: two bags cover 450 × 500 mm at 75 mm thick, which works out at about 8.4 litres a bag, or about 119 bags per m³ (Cement Australia product sheet (PDF)). There's more on this, with the sum for each product, in how many 20 kg bags of concrete make a cubic metre.

Common mistakes

• Millimetres as metres. 100 mm is 0.1 m, not 0.01 m. A slip there is a tenfold error. Entering millimetres, as this calculator asks, avoids it.
• Radius for diameter. Using the full 300 mm as the radius gives four times the concrete: 0.3² = 0.09 against 0.15² = 0.0225 in the area sum. Enter the diameter; the calculator halves it.
• Outside the formwork. Measure to the inside face of the form boards. That's the space the concrete fills.
• Nominal depth on an uneven base. A base that's 20 mm low in places adds 20% to a 100 mm slab where it's low. Check the depth in several spots, or allow more than 10% on rough ground.
• Ordering the exact volume. Running short mid-pour is worse than a little left over. Order the figure with the 10% in it.
• Holes that aren't round or straight. A hole dug by hand often belles out at the bottom. Measure the width at the bottom too, and use the larger figure if in doubt.

Questions people ask

What this calculator doesn't do

• Boxes and round holes only. No thickened edges, internal beams, steps, slopes or tapers: work those out as separate boxes and add them.
• It doesn't subtract posts, pipes, reinforcement or anything else set in the concrete.
• It doesn't choose a concrete strength, mix, slab thickness or footing size. Those come from the drawings, the code and your council.
• The 10% is fixed, as in the app. Bag counts are only as good as the makers' approximate yields.
• Millimetres and cubic metres only, here. No prices.

Concrete in MeisterCalc

The same maths is in MeisterCalc's Concrete calculator, with a Slab tab (use it for strip footings too) and a Holes tab, a live diagram, and an optional cost: enter a price per m³ and set the wastage from 0 to 30%. Home ▸ Concrete. For a price of 200 per m³, the 1.2 m³ slab costs 1.2 × 200 = 240, and 264 with 10% wastage. It's one of three MeisterCalc calculators that are free to use, along with Stairs and Check Square, and it works offline on site, on Android 7.0 or later. The other 12 calculators unlock with a weekly subscription that starts with a 3-day free trial, or with a one-time lifetime unlock; Google Play shows the prices in your currency. There's no iPhone version yet. More on the app in is there a free concrete calculator app.

Related guides

• How much concrete do I need for a slab?
• How much concrete do I need for a post hole?
• How many 20 kg bags of concrete make a cubic metre?
• Is there a free concrete calculator app?
• More guides on the blog