Concrete Bag Calculator
Find how many 80-lb bags of concrete mix you need for a slab, footing, or post hole, plus total cost at your price per bag.
How this calculator works
For small pours — footings, post holes, patches — concrete is mixed from bags rather than delivered by truck. This calculator converts your project’s cubic yardage into cubic feet, then divides by the 0.6 cubic feet an 80-lb bag yields once mixed.
Formula: Cubic yards = (Length ft × Width ft × Depth ft) ÷ 27. Bags = (Cubic yards × 27) ÷ 0.6.
Worked example
A 10 ft × 10 ft slab at a 4-inch depth:
- Depth in feet: 4 ÷ 12 = 0.333 ft
- Cubic yards: (10 × 10 × 0.333) ÷ 27 ≈ 1.235 yd³
- Bags: (1.235 × 27) ÷ 0.6 ≈ 55.56 bags
- At $6/bag, that’s $333.33 before waste.
Coverage reference
| Slab size | Depth | Bags needed (80-lb) |
|---|---|---|
| 4 ft × 4 ft | 4 in | ~8.9 bags |
| 10 ft × 10 ft | 4 in | ~55.6 bags |
| 12 ft × 12 ft | 4 in | ~80.0 bags |
How to measure your project
Measure the pour footprint in feet and the depth as the actual thickness of the concrete, not including any gravel base beneath it. For post holes, measure the hole’s diameter and depth, then treat it as a small rectangular volume by using the diameter as both length and width — this slightly overestimates but keeps you from coming up short.
Frequently asked questions
How much concrete does one 80-lb bag make?
An 80-lb bag of concrete mix yields about 0.6 cubic feet once mixed with water. This calculator converts your project's volume into cubic feet and divides by 0.6 to get bag count.
At what point should I switch from bags to ready-mix?
Once a pour approaches 1 cubic yard — roughly 45 bags of 80-lb mix — ready-mix delivery is usually cheaper and far less labor, since mixing 45+ bags by hand or in a small mixer is a full day's work. Bags make more sense for footings, post holes, and small patches.
Do 60-lb bags work the same way?
60-lb bags yield about 0.45 cubic feet each, less than the 0.6 cubic feet from an 80-lb bag, so you'd need more of them for the same volume. This calculator assumes 80-lb bags; scale the bag count up by roughly 1.33x if you're buying 60-lb bags instead.
Should I round the bag count up?
Yes — bags are a whole-unit purchase, so round any fractional result up to the next full bag. It's also worth buying one or two extra bags beyond that, since a bag or two short mid-pour is a bigger problem than a leftover bag.
How much water does each bag need?
Most 80-lb bags call for roughly 3 quarts (about 0.75 gallons) of water per bag, though this varies by brand — check the bag for the manufacturer's exact ratio, since too much water weakens the cured strength.
Estimates only. Verify quantities with your supplier before purchasing.