How to Calculate Rebar for a Foundation Wall in 5 Steps

Rebar for a foundation wall in one line

Rebar for a foundation wall is wall length, times 12, divided by spacing, plus waste, times bar length. A 40 ft wall with bars every 24 inches needs 20 bars exact; with 10% waste the order is 22 bars, or 176 feet of bar. The rest of this guide is about getting those three inputs honest: the wall length, the spacing, and the waste allowance.

Step 1: Measure the wall length

Measure the length of the wall the bars run along, in feet, edge to edge, on the wall face itself — not the footing, since the bar count follows the wall length, and a wall face is what the bars reinforce. The worked example is 40 ft long, a normal foundation wall run. If the wall has an opening the bars skip, measure it too: it comes off the length in step two. A wall that is 40 ft long is the foundation of everything that follows, and a length measured gross is the first way the order goes wrong.

Step 2: Turn spacing into a bar count

Bars are spaced in inches, and the wall is measured in feet, so convert before dividing: multiply the wall length by 12 to get inches, then divide by the spacing. A 40 ft wall is 480 inches; at 24 inches apart that is 480 divided by 24, or 20 bars exact. Keep the spacing honest — read it off the structural drawing, not off a habit: at 12 inches the same wall needs 40 bars, double the order, from one drawing figure. If the division leaves a fraction, round up: the wall still needs that last bar.

Step 3: Add waste for cuts and laps

Bars are cut, and bars are lapped where they join, so add waste before ordering: 10% for a straight run takes 20 bars to 22. A wall with many corners and laps wants 15%, which would take the same wall to 23 bars before rounding. Waste is not spare stock — it is the bar the job consumes in cutting and lapping, and a wall ordered without it comes up short at the last bay, with the concrete booked and the steel short.

Step 4: Total the bar length

Multiply the bars by the bar length to get the total feet ordered: 22 bars at 8 feet each is 176 feet of bar. This total is what the merchant prices if the bars are sold by the foot, and it is what the truck carries — a bar count alone does not tell the merchant how much steel is coming. Bar length changes only this total, never the count: 22 bars at 10 feet is 220 feet, but it is still 22 bars.

Step 5: Order whole bars, and count the extras

Order 22 bars, since bars are sold by the piece and 22 is already whole — the rounding happened at the waste step, not at the merchant. Then count what bars do not cover: the horizontal bars, the corner bars, the chairs that hold the bars off the footing, and the ties that bind them — none of these come in the bar count. Keep a few spare bars after the job: they patch a miscut, and a matching bar size may not be on the next delivery.

Rebar in numbers

Suppose the wall is 40 ft long, bars every 24 inches, 8 ft bars, 10% waste. Inches: 40 × 12 = 480. Bars exact: 480 ÷ 24 = 20. With waste: 20 × 1.10 = 22 bars. Total length: 22 × 8 = 176 ft. Order: 22 bars of 8 ft, plus chairs and ties bought separately.

Common mistakes

The most common mistake is using the spacing from habit instead of the drawing, which doubles or halves the order at a stroke. The second is measuring the footing instead of the wall, which counts bars for a length the wall never runs. The third is skipping the waste allowance, which under-orders by exactly the amount the cutting and lapping consume. The fourth is counting the bar length as the bar count, which confuses feet with pieces. The fifth is forgetting the second layer, the corners and the ties, which are separate counts, not waste.

Worked example, start to finish

A wall, 40 ft long, bars every 24 inches, 8 ft bars, 10% waste. Inches: 480. Bars exact: 20. With waste: 22 bars. Total length: 176 ft. Result: order 22 bars of 8 ft, plus chairs and ties bought separately, and keep a few spare bars for miscuts. If the same wall used 12 inch spacing instead, the exact count would be 40 bars and the order 44 with waste — the spacing alone would add 22 bars, which is why step two, reading the drawing, matters most.

What changes the answer most

The spacing changes the answer most: moving from 24 inches to 12 inches doubles the whole order at a stroke, since it divides every inch of wall. The wall length is second: length moves the count, but only across a bar rounding does the shop list change. The waste allowance is third: 10% is small beside the spacing, but it is the difference between a finished last bay and an open one. The bar length is fourth, and it changes only the total feet, never the count. If the budget is tight, build a shorter wall before cutting the waste allowance.

Frequently asked questions