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Concrete Bag Calculator

Convert any small pour into bag counts — 40, 60 or 80 lb bags — for slabs, footings, fence posts and repairs.

Concrete Bag Calculator

How many 40, 50, 60 or 80 lb (20–40 kg) bags of concrete you need for slabs, footings or post holes.

How this is calculated

Volume (ft³) ÷ bag yield. Yield ≈ 0.0075 ft³ per lb of mix (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³). Post holes: (π × (hole ÷ 2)² − post²) × depth.

Enter what you are filling — a small slab, a footing, or a set of post holes (with the post size deducted) — pick your bag size and get the exact bag count, total weight, pallets, mixing water and cost.

Results are estimates for planning purposes only. Real-world quantities depend on site conditions, materials and workmanship. Always verify measurements, follow local building codes and confirm quantities with your supplier or a qualified professional before ordering or building.

About the Concrete Bag Calculator

For small jobs — a fence post, a mailbox base, a driveway patch — a ready-mix truck makes no sense and bagged concrete is the answer. But each bag size yields a different volume. This calculator converts any small pour into an exact bag count for 40, 60 and 80 lb bags, and estimates the water you will need.

How to use

  1. Choose the shape: slab or pad, round post hole, square footing, concrete tube or custom volume.
  2. Enter the dimensions — for a post hole, the diameter and depth of the hole.
  3. Pick your bag size: 80 lb for best value, 60 or 40 lb if you are carrying them far or working alone.
  4. Set waste to 5 percent for formed pours, 10 percent for hand-dug holes, which always end up oversized.
  5. Check the water estimate before you start — you cannot pause mid-pour to fetch more.

Understanding your results

The bag count rounds up to whole bags per size, so you can compare the 80 lb option against lighter bags directly. Total volume in cubic feet lets you sanity-check against the hole or form you dug. The water figure is approximate — always add water gradually, since you cannot take it back out.

How the calculation works

Each shape uses its volume formula: slabs are length times width times thickness; round holes are pi times radius squared times depth. Volumes convert to cubic feet, waste is added, and the total divides by yield per bag: about 0.6 cubic feet for 80 lb, 0.45 for 60 lb, 0.3 for 40 lb. Water runs roughly 3 quarts per 80 lb bag.

Concrete Bag Calculator FAQs

About 45 eighty-pound bags, 60 sixty-pound bags or 90 forty-pound bags. One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.6 cubic feet, so 27 divided by 0.6 equals 45. If your project needs more than about 30 to 40 bags, price a ready-mix delivery — it is usually cheaper and far less labor.
About two 80 lb bags for a typical post hole — 10 inches in diameter and 30 inches deep, which is roughly 1.4 cubic feet before the post displaces some volume. Hand-dug holes always end up wider than planned, so the calculator default of 10 percent waste covers the real world. Set the post first, then pour the mix around it.
Roughly 3 quarts per 80 lb bag, added gradually while mixing — but always check the bag, since mixes vary. The right consistency is like thick oatmeal: it should hold its shape without crumbling or running. Too much water is the number one DIY mistake — it can cut the final strength by 20 percent or more.
Fast-setting mix hardens enough to hold a post in 20 to 40 minutes, versus several hours for standard mix, and it can be poured dry into the hole with water added on top. Standard mix is cheaper per bag and better for slabs and footings where you need working time. For fence posts, fast-setting saves an entire day of bracing.
Yes — one or two 80 lb bags at a time is the practical limit for a standard wheelbarrow. Mix dry first, then add water gradually while turning the mix with a hoe. For more than about 6 to 8 bags, rent a portable mixer: hand-mixing that much is exhausting and the first batches start setting before the last are mixed.
Nabil Sharif, founder of CalcDune

Written by

Nabil Sharif

Founder of CalcDune — a free library of 137 online calculators for finance, health and everyday math. Every calculator shows the formula behind its result, so you can see exactly how your numbers are worked out.

More articles by Nabil →

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