Rebar Calculator

Before you count a single bar, decide how far apart the rebar will sit. That one spacing number drives everything else, from the number of bars you buy to how tough the slab actually becomes at the end. Tight spacing costs more steel but buys a slab that refuses to crack when a truck drives over it, while wide spacing saves money on a patio that never carries anything heavier than a grill.

Rebar is sold in straight 20 foot bars, so the real task is laying out a grid and then cutting that grid into 20 foot lengths. You are not pouring concrete yet. You are planning a wire-tied mat that will sit inside the wet pour, and the mat only works if you know the spacing before you tie the first tie wire.

Start with the spacing

Spacing is measured on center, which means from the middle of one bar to the middle of the next. A 12 inch grid puts a bar every foot, an 18 inch grid every foot and a half, and a 24 inch grid every two feet. Pick 18 inches for most footings and lightly used patios. Step down to 12 inches for driveways, garage slabs and anything that sees vehicle traffic. Stay at 24 inches only for narrow footings, sidewalk edges and garden beds where loads are light.

The spacing choice is worth pausing on because it is cheap to change on paper and expensive to change after the concrete truck arrives. Twelve inch spacing uses nearly twice the steel of 24 inch spacing, yet it is the difference between a driveway that cracks in year three and one that quietly lasts decades.

Lay out the two directions

A slab almost always wants bars running in both directions. One set of bars runs the length of the pad, and a second set runs across the width, tied together at every crossing to form a grid. This is what keeps the concrete from opening up cracks no matter which way the ground shifts under it.

For a 4 inch residential slab, that grid usually sits as a single layer centered in the pour. Deeper 6 inch footings and driveways sometimes call for two layers, one near the top and one near the bottom, but that is a commercial detail rather than a backyard one. Support the grid on plastic rebar chairs so the bars float about two inches above the sub-base and end up buried inside the concrete after the pour.

Count the 20 foot bars

Once the spacing and the slab size are set, the bars per direction follow naturally. A 10 foot wide slab at 18 inch spacing needs roughly 7 bars across the width, each running the length of the pad. Flip the grid and you get the same count running the other way, then you tie the two layers together.

Because bars come in fixed 20 foot lengths, a slightly longer slab rarely needs more whole bars than a shorter one. A 10 foot by 10 foot pad at 18 inch spacing lands around 7 twenty foot bars in total once both directions are rolled up, and the leftover offcuts are enough to patch corners. A 20 foot driveway at 12 inch spacing climbs past 30 bars, which is exactly why tighter spacing surprises people at the lumber yard.

Rebar versus wire mesh

The two-dimensional grid of rebar is not the only way to reinforce concrete. Wire mesh, the welded square grid sold in rolls or sheets, is lighter and cheaper, and it gets laid under patios and thin sidewalks all the time. It controls cracking, but it bends and settles more than solid rebar, so it does not hold up the way a driveway needs.

Reach for rebar when the slab carries cars, trucks or a heavy hot tub, and when the soil under it is prone to shifting. Reach for wire mesh on a 4 inch patio that only sees foot traffic. The cost gap is real, but the failure gap is real too, and a cracked driveway is a permanent fixture in the yard while the savings on mesh are a one-time event.

A layout to copy

Say you are pouring a 10 by 10 foot backyard pad at 18 inch spacing. Run number 4 rebar both ways, keep one layer held on chairs, and plan on about 7 twenty foot bars plus a couple of spares for offcuts. Tie the crossings with short pieces of tie wire, lift the mat into the middle of the pour, and you have done the reinforcement correctly.

Measure twice, tie once. The bars are cheap, the labor is the expensive part, and a grid that is off by a few inches under fresh concrete is not something you get a second chance at.

See also the concrete pad calculator for the slab volume and the post hole concrete calculator for the footings that hold the edges.

A 10 by 10 foot slab at 18 inch spacing needs about 7 twenty foot bars of number 4 rebar.