Concrete Calculator
Premix bags needed
Bag yields are the manufacturer-published figures (0.30/0.45/0.60 ft³ for 40/60/80 lb bags — the same numbers printed on most premix bags). Buy 5–10% extra to cover spillage and an uneven subgrade.
Free concrete calculator for slabs, footings, walls, and round post holes — get the cubic yards needed and, unlike most calculators, exactly how many 40/60/80 lb premix bags to buy.
How it works
- Choose rectangular (slab, footing, wall) or round (column, post hole)
- Enter your dimensions
- See the volume needed in cubic yards and feet, plus the exact bag count for 40/60/80 lb premix
Frequently asked questions
How many bags of concrete do I need?
This is calculated directly from the volume using each bag size's manufacturer-published yield (40 lb = 0.30 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³ of mixed concrete) — the same numbers printed on most premix bags.
Should I buy extra?
Yes — buy 5-10% more than the calculated amount to cover spillage, an uneven subgrade, or minor measurement error. Running short mid-pour is a much bigger problem than a small leftover.
What about fence post holes?
Use the round mode with the hole's diameter and depth, and set "Number of holes" to however many posts you're setting — it multiplies the volume and bag count automatically.
Related tools
From the blog
Update history
- 2026-09-03 The shape toggle now shows an icon for each option (slab vs column).
- 2026-09-01 Launched: concrete calculator for rectangular and round pours, with cubic yard/foot volume and 40/60/80 lb premix bag counts. Entirely client-side.