For fence posts, deck posts, gate posts, and mailbox posts. Type in your hole size and post size, and this page does the math out loud — every step, in plain words — so you can see exactly where the bag count comes from.
Call 811 (or visit call811.com) at least a few working days before you dig. Buried gas, electric, water, and phone lines get marked for free. Hitting a buried line can kill. Never skip this.
One more thing: deck footings that hold up a deck or a roof are structural — many towns require a permit and an inspection for them. A fence post is usually simpler. Check with your local building office before you dig footings for anything that carries a roof or a deck.
Change any number and the math below updates right away.
Lumber sizes are sneaky. A 4×4 really measures 3.5 inches across, and a 6×6 really measures 5.5 inches across. The buttons above fill in the real size, because the real size is what takes up room in the hole. If your post is a different size, just type its true width.
A round hole is a cylinder. Halfway across the hole is the radius. We turn inches into feet (divide by 12), then multiply:
The post sits in the hole, so part of the hole is already full of wood. That part needs no concrete. Some calculators skip this step and over-count. We subtract the post.
Divide the concrete you need by how much one bag makes. Bags only come whole, so we always round up — a part bag still means buying a bag.
Bag yields are the yields printed on Quikrete bags — check the bag you buy, brands vary a little.
The numbers you type stay on this page, in your browser. Nothing is sent anywhere, and there is no sign-up.
Say you are setting a 4×4 fence post in a hole 10 inches wide and 30 inches deep. Here is the whole job on paper — the same math the calculator above does.
The hole is 10 inches wide, so the radius (halfway across) is 5 inches. In feet: 5 ÷ 12 = 0.4167 feet. The depth is 30 inches = 2.5 feet.
3.1416 × 0.4167 × 0.4167 × 2.5 = 1.36 cubic feet
A real 4×4 measures 3.5 inches across, not 4. In feet: 3.5 ÷ 12 = 0.2917 feet. The post fills that much of the hole, for the full 2.5-foot depth:
0.2917 × 0.2917 × 2.5 = 0.21 cubic feet of post
1.36 − 0.21 = 1.15 cubic feet of concrete per hole
80 lb bag: 1.15 ÷ 0.60 = 1.92 → 2 bags
60 lb bag: 1.15 ÷ 0.45 = 2.56 → 3 bags
50 lb fast-setting bag: 1.15 ÷ 0.375 = 3.07 → 4 bags
So one post like this takes 2 bags of 80 lb, or 3 bags of 60 lb, or 4 bags of 50 lb fast-setting mix. Ten posts? Multiply by ten: 20, 30, or 40 bags.
These are the guidelines experienced fence builders pass along. Each one is a rule of thumb, not a code rule — your town's code and your ground conditions come first.
For a 4×4 post (3.5 inches across), that means a hole roughly 10 to 12 inches wide. The extra room is where the concrete goes — it is the concrete collar around the post that does the holding.
An 8-foot post usually goes about 2½ to 3 feet in the ground. In cold areas, the bottom of the hole should reach below the frost line, so the ground cannot heave the post up in winter. Your local building office or a local fence supplier can tell you your frost depth; it is different in every state.
A few inches of gravel at the bottom of the hole helps water drain away from the end of the post instead of pooling there. It is a common practice, and it means the post end is not sitting in a puddle. (If you add gravel, remember the post sits on top of it — dig the hole that much deeper.)
Bag yields (0.60, 0.45, and 0.375 cubic feet per bag) are as printed on Quikrete concrete mix bags. Safe-digging guidance — call 811 before you dig — is from call811.com, the national safe-digging site.
Checked November 2026.