20 kg Concrete Calculator
This 20 kg concrete calculator converts your pour's dimensions straight into a bag count: one 20 kg bag yields about 11.0 litres (0.0110 m³) of mixed concrete, so the tool divides your volume by that yield and rounds up with waste.
How to Use This Calculator
- Enter your dimensions — meters for lengths, centimeters for thickness.
- Adjust the options — waste percentage, bag size and your local concrete price.
- Read your results — volume, bags, weight and cost update live as you type.
The Formula
Bags = volume ÷ 11.0 litres (0.0110 m³) per bag, rounded up (+5–10% waste) Worked example: A 3 m × 3 m slab at 10 cm = 0.9 m³ → 0.9 ÷ 0.0110 ≈ 82 × 20 kg bags before waste.
The 20 kg bag is the standard DIY size in Australia and New Zealand. A concrete calculator 20kg bags shoppers rely on needs the right yield — about 11 litres per bag mixed, so ~91 bags per cubic meter. Don't confuse concrete mix with cement: a 20 kg bag of cement goes much further when batched with ballast.
Reference numbers for 20 kg bags: 70 bags make one cubic yard, 91 bags make one cubic meter. Keep those in your head at the store and you can sanity-check any estimate.
Bagged concrete makes sense up to roughly one cubic yard. Beyond that, price out ready-mix — mixing 45+ bags by hand is a full day of hard labor and the per-yard cost usually favors the truck.
Frequently Asked Questions
Common questions about the 20 kg concrete calculator.
How much concrete does a 20 kg bag make?
About 11.0 litres (0.0110 m³) of mixed concrete, per standard manufacturer yield data.
How many 20 kg bags in a cubic meter?
91 bags per cubic meter (1 ÷ 0.0110 m³, rounded up).
How many 20 kg bags for a fence post?
A 300 mm hole 600 mm deep needs ≈ 0.042 m³ — about 4 bags per post.
Should I add extra bags?
Yes — buy 5–10% more than calculated. Unopened bags return easily; a mid-pour shortage doesn't.
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