Enter the size of your slab to see the concrete you need, with the formula and every assumption shown.
Concrete slab job sheet · ProjectTallyKit.com
Your estimate
Fix the highlighted field to update the estimate. The figures below are for your last valid entry.
Buy 62 × 80-lb bags
Exact amount with waste: 1.358 yd³ (36.67 ft³)
- Area 100 ft² at 4 in thick
- 10% waste added once, to the total
- Rounded up to whole bags. Each bag weighs 80 lb.
Or about 1.36 yd³ of ready-mix concrete
This is a material estimate, not a structural design. Thickness, reinforcement, sub-base and loads need site-specific advice.
| Bag size | Bags | Weight | Left over | Cost |
|---|---|---|---|---|
| 40-lb | 123 | 4,920 lb | 0.23 ft³ | — |
| 50-lb | 98 | 4,900 lb | 0.08 ft³ | — |
| 60-lb | 82 | 4,920 lb | 0.23 ft³ | — |
| 80-lb | 62 | 4,960 lb | 0.53 ft³ | — |
Bags per cubic yard, rounded up: 90 × 40-lb, 72 × 50-lb, 60 × 60-lb, 45 × 80-lb. Yields from the QUIKRETE Concrete Mix No. 1101 data sheet.
Least concrete left over: 5 × 50-lb + 58 × 80-lb leaves 0.01 ft³, against 0.08 ft³ with 98 × 50-lb.
Cost comparison
- Add your own bag or ready-mix prices under “Buying options and prices” to compare costs. Prices are never filled in for you.
What if
| Thickness | Order | 80-lb bags | Cost |
|---|---|---|---|
| 3 in | 1.02 yd³ | 46 | — |
| 4 in | 1.36 yd³ | 62 | — |
| 5 in | 1.70 yd³ | 77 | — |
| 6 in | 2.04 yd³ | 92 | — |
Show the math with my numbers
- Section 1: 10 ft × 10 ft = 100 ft²
- Volume: 100 ft² × 4 in ÷ 12 = 33.33 ft³
- With waste: 33.33 ft³ × 1.1 = 36.67 ft³
- In cubic yards: 36.67 ft³ ÷ 27 = 1.358 yd³ (1.038 m³)
- Bags: 36.67 ft³ ÷ 0.6 ft³ per 80-lb bag = 61.11, rounded up to 62
Shopping list
- Concrete mix, 80-lb bags62 bags (4,960 lb)
Calculations run in your browser. Your measurements are not sent to us.
How this estimate is calculated
This is a material estimate, not a structural design. Thickness, reinforcement, sub-base and loads need site-specific advice.
Before you buy
- Measure length and width inside the forms. Measure the depth in several places; on uneven ground, use the depth readings option.
- Read the yield and weight on the bag you will buy and change them under “Bag options” if they differ.
- For ready-mix, ask the supplier about minimum loads, short-load and delivery fees, and how they round orders; enter them to compare costs.
- Plan how you will move and mix the bags: the result shows the total weight.
- This estimate covers concrete and, if you tick them, sub-base, vapor barrier and edge length. It does not cover reinforcement, forms, tools or finishing supplies.
- Check permits, local code and utility locates before you dig or pour.
Formula
- Area: rectangle L × W; circle π × (D ÷ 2)²; ring π × ((D ÷ 2)² − (d ÷ 2)²); triangle by Heron’s formula from its three sides. Sections are added up.
- Volume (ft³) = area (ft²) × thickness (in) ÷ 12
- Order volume = volume × (1 + waste ÷ 100), applied once to the total
- Cubic yards = cubic feet ÷ 27
- Bags = order volume ÷ bag yield, rounded up to a whole bag
Worked example
- Section 1: 10 ft × 10 ft = 100 ft²
- Volume: 100 ft² × 4 in ÷ 12 = 33.33 ft³
- With waste: 33.33 ft³ × 1.1 = 36.67 ft³
- In cubic yards: 36.67 ft³ ÷ 27 = 1.358 yd³ (1.038 m³)
- Bags: 36.67 ft³ ÷ 0.6 ft³ per 80-lb bag = 61.11, rounded up to 62
| Case | Before waste | With waste | Exact bags | Bags to buy |
|---|---|---|---|---|
| Default example: 10 × 10 ft, 4 in thick, 10% waste | 33.33 ft³ | 36.67 ft³ | 61.11 × 80-lb | 62 × 80-lb |
| 12 × 12 ft, 4 in thick, 10% waste | 48 ft³ | 52.8 ft³ | 88 × 80-lb | 88 × 80-lb |
| Circle 10 ft across, 4 in thick, 10% waste | 26.18 ft³ | 28.8 ft³ | 48 × 80-lb | 48 × 80-lb |
| 10 × 10 ft, depths 3.5, 4 and 4.5 in (average 4 in), 10% waste | 33.33 ft³ | 36.67 ft³ | 61.11 × 80-lb | 62 × 80-lb |
This page recalculates each case with the same formulas as the calculator, and the same cases run as automated tests of the code.
| Bag size | Yield | Exact | Rounded up |
|---|---|---|---|
| 40-lb | 0.3 ft³ | 90 | 90 |
| 50-lb | 0.375 ft³ | 72 | 72 |
| 60-lb | 0.45 ft³ | 60 | 60 |
| 80-lb | 0.6 ft³ | 45 | 45 |
Rounding
Everything is calculated at full precision. Bags are rounded up to whole bags (after removing floating-point noise, so 52.8 ft³ of 0.6 ft³ bags is 88, not 89). Cubic yards and cubic metres are never rounded up to whole units; ready-mix is only rounded if you set your supplier’s order steps.
Constants and sources
What this estimate does not cover
Not included: reinforcement, control joints, labor, tools, delivery rules, taxes, and anything about whether a slab of this size and thickness is suitable. Ask a local professional or your building department about design and code.
Last reviewed October 12, 2026. Next review due April 12, 2027.