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Concrete Calculator — Slabs, Footings, Post Holes & Bags

Measure once, order once. Pick the shape, type your dimensions in inches, and get cubic yards, cubic feet, and the number of 40, 60 or 80 lb bags to buy — free, instant, and nothing leaves your browser.

Concrete Calculator — slabs, footings, columns & post holes

in
in
in
×
0.01
cubic yards
0.2
cubic feet
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cubic meters
Bags needed (buy count)
  • 40 lb bag1
  • 60 lb bag1
  • 80 lb bag1

Yields used: 40 lb ≈ 0.30 ft³ · 60 lb ≈ 0.45 ft³ · 80 lb ≈ 0.60 ft³ after mixing. Add 5–10% for spillage before ordering.

Crushed-stone / gravel equivalent ≈ 0.01 tons at ~1.4 tons per yard. All math runs locally in your browser.

Slab & footing math — ready mix or bagged

A slab's volume is just area × thickness, but the units bite: dimensions live in feet-and-inches while ready-mix trucks sell cubic yards (1 yd³ = 27 ft³). The calculator converts for you; the classic shortcut for 4-inch slabs is square feet ÷ 81 = yards. Footings follow the same math — enter the trench length as “length” and the footing width and depth, then use Quantity for multiple runs.

Always add 5–10% for spillage and uneven ground before ordering. Ready-mix suppliers typically have a 1-yard minimum and sell in quarter-yard increments after that; a short load fee applies to small pours.

Post holes & fence posts

Fence-post concrete is a cylinder problem: hole diameter and depth decide everything, and the buried post displaces part of it. The rules worth memorizing: hole diameter 3× the post width, depth ⅓–½ of the exposed post height, and about one 50 lb bag per post at typical sizes. For a 6-foot privacy fence that means a 12-inch hole 24 inches deep — roughly 1.5 bags per post before subtracting the post itself.

Sonotubes & round columns

Cylindrical forms (Sonotube being the household name) fill to π × (diameter ÷ 2)² × height. Common sizes: an 8-inch-diameter tube 4 feet tall takes ≈ 1.4 ft³ (three 60-lb bags); a 12-inch × 4-ft tube takes ≈ 3.1 ft³ (six 60-lb bags). Always pour a few inches of gravel in the base first and mound the top for water runoff.

By bag spec — Sakrete & Quikrete yields

Bag counts above follow the printed yield specs used by the major brands (Sakrete, Quikrete and house labels are functionally interchangeable at the same weight): 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³ of mixed concrete. Check the bag you actually bought — some “high-strength” 80 lb formulations yield slightly less (0.60 vs 0.66 ft³) — and when a table asks for bags of a specific brand, convert by volume, not by count, since brand-to-brand bag sizes differ.

Crushed concrete & gravel bases

Slabs usually sit on compacted gravel, and crushed concrete is the budget-friendly version of it. Stone sells by the ton — roughly 1.4 tons per cubic yard — so the same volume math applies: area × depth ÷ 27 × 1.4. A 4-inch gravel base under that 10×10 slab is about 1.2 yards ≈ 1.7 tons of crushed stone. The tonnage line under the calculator tracks your inputs live.

Concrete Calculator FAQ

How many bags of concrete do I need?

Divide the total cubic feet by what one bag yields after mixing: a 40 lb bag yields ≈ 0.30 ft³, a 60 lb bag ≈ 0.45 ft³, an 80 lb bag ≈ 0.60 ft³. The calculator above does this for your exact dimensions and rounds up to whole bags. Example: a 4-inch-thick 10×10 ft slab is 33.3 ft³ ≈ 56 eighty-pound bags — a case where ordering ready-mix delivery makes more sense.

How do I calculate cubic yards of concrete?

Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 — one cubic yard is 27 cubic feet. For a 4-inch slab, the shortcut is square footage ÷ 81: a 10×10 pad (100 sq ft) needs ≈ 1.23 yards.

How much concrete per fence post?

Rule of thumb: the hole should be 3× the post width and 1/3 to 1/2 the post's above-ground length. A 4×4 post in a 10-inch-diameter, 24-inch-deep hole needs about one 50 lb bag (0.37 ft³ of concrete) — the Post Hole mode above subtracts the post volume for you.

Should I order ready-mix or bags?

Bags win under roughly 1 cubic yard — no delivery fee, no excess, no wheelbarrow race against a curing truck. Above ~1.5 yards, ready-mix is usually cheaper and always faster. Between the two, price out both with this calculator first.