Concrete calculator: volume, waste and bags
Concrete math is length×width×thickness plus waste, then bags: a 5×3 m slab 10 cm thick is 1.5 m³, 1.65 m³ with 10% waste, about 127×25 kg or 80×40 kg bags. Waste covers spillage and uneven subgrade, and thickness in cm must become meters first. This calculator is built for fast, private, offline use in your browser. Every formula runs locally and is verified against official sources. You enter the key numbers, see the result instantly and check the steps.
TL;DR
- Add waste percent, divide by bag yield to get whole bags rounded up.
- Formulas: m³ = L×W×(cm÷100) × (1+waste) | bags = m³÷yield
- Example 1: 5×3 m slab, 10 cm thick, +10% waste — Volume: 5×3×0.10 = 1.500 m³
How it works
Volume = length×width×(thickness cm ÷ 100). Add waste percent, divide by bag yield to get whole bags rounded up. Start with clean data, keep the same units and follow the order in the formula. The tool does the arithmetic, you choose rate, period or factor. A second example with different numbers shows how the result scales.
The math
5×3×0.10=1.500 m³ +10% = 1.650 m³, so 127×25 kg or 80×40 kg bags. Thin check: 2×2×0.05=0.200 m³, so 16×25 kg bags.
Always cm÷100 first. Bags round up, never down.
Examples
Example 1: 5×3 m slab, 10 cm thick, +10% waste
- Volume: 5×3×0.10 = 1.500 m³
- With waste: 1.500×1.10 = 1.650 m³
- Bags: 127×25 kg or 80×40 kg
Example 2: 2×2 m patch, 5 cm thick, no waste
- Volume: 2×2×0.05 = 0.200 m³
- Bags: 16×25 kg
- One extra bag covers spillage
When to use it
✦ Garden slab
Order concrete for a shed base: cubic meters with waste and bag count from one input.
✦ Fence post footings
Small pours: volume for a thin patch and the smallest bag count that covers it.
⚠ Common mistakes
- Centimeters as meters: 10 cm is 0.10 m, not 10 m, a hundredfold error.
- Zero waste: always add 5-10%, the subgrade is never perfectly level.
- Rounding bags down: 15.4 bags means buying 16.
FAQ (6)
How many bags per m³?
About 77×25 kg per cubic meter at this yield (127 bags for 1.65 m³).
What waste percent?
10% for slabs, 5% for neat formwork.
Do you save the numbers I type for area, volume or conversions?
Not at all. Every conversion — kg to lbs, area, fraction math — runs locally. No form data is posted anywhere. You can convert 1,500 kg to lbs on a shared computer and walk away knowing nothing was logged.
Will it work without internet in class?
Yes — that's why students like it. Load once on Wi-Fi, then fraction, base-converter or standard deviation work offline in class or in an exam hall without signal. It's just math in JavaScript, no server round-trip.
How accurate are the math results?
We use standard definitions (e.g. 1 kg = 2.20462 lbs, BIPM SI) and double-check edge cases via automated tests. For homework or site measurements it's more than enough, but for certified engineering or lab work, cross-check with the primary standard.
Can I share a calculation with a classmate?
Tap 'Share Link' — it puts the inputs in the URL. Your classmate opens it and sees the same 3/4 → 0.75 → 75% breakdown instantly, without retyping.
Sources & References
How this calculator is maintained
This calculator is written and maintained by Davide Coronese, independent developer and founder of CalcTracker.
Methodology: every formula is checked against official publications and validated by an automated test suite before release. Health, tax and finance pages cite the exact sources they rely on.
Found an error? Contact us — verified corrections ship within 24 hours.