How to Calculate Rebar for a Strip Footing, Stirrups Included

2026-10-14

To calculate rebar for a strip footing, add up three things. First, the longitudinal bars: the number of bars in the section times the total footing length, plus an allowance for laps and corners. Second, the stirrups: the footing length divided by their spacing, rounded up, times the cut length of one closed stirrup. Third, convert both totals from metres to kilograms using the nominal mass per metre of each bar size. Take a 50 m footing, 400 mm wide and 600 mm deep, with four 12 mm bars and 8 mm stirrups every 300 mm. With a 10% allowance it comes to 318.8 kg of steel.

Most online guides stop after the first step. The stirrups get a line or two, and some guides leave them out entirely. In the example above they are more than a third of the weight, and getting them right depends on two details these guides rarely mention: concrete cover and hooks.

How to calculate rebar for a footing, step by step

Here is the full method, in the order you should do it.

• Total footing length. Measure the outer perimeter and add every interior load-bearing wall that sits on its own strip. In the example, that's 50 m.
• Longitudinal bars. Multiply the length by the number of bars in the cross-section, counting top and bottom together. Four bars over 50 m is 200 m. Add an allowance for laps, corner bars and offcuts. At 10% that's another 20 m, so you need 220.0 m.
• Stirrups. Divide the footing length by the spacing and round up. At 300 mm spacing that's 167 stirrups. Each one is a closed rectangle that follows the section, minus the concrete cover on every side, plus a hook allowance.
• Weight. Multiply each total length by the nominal mass of one metre of that bar size. Here that gives 195.4 kg of 12 mm bar and 123.5 kg of 8 mm stirrups.

Each step is simple arithmetic. The mistakes come from the inputs, and from what people leave out.

Stirrups: the third of the weight that guides skip

One of the best-ranked footing guides in Google calculates continuous footings as bars times perimeter, plus 10%, and stops there. Another lets you enter tie spacing but never shows how it counts or sizes the ties. A metric calculator that ranks for the same topic handles a footing only as one total run length, so you'd have to work out the links separately.

In the example, the 167 stirrups add up to 312.6 m of 8 mm bar, or 123.5 kg. That's more than a third of the 318.8 kg total. If you price only the longitudinal steel, you'll be short by that much.

The spacing matters as much as the bar size. Builders' worked examples use anywhere from 300 mm to 500 mm between stirrups for the same kind of house footing. Going from 500 mm down to 300 mm multiplies the number of stirrups by 1.67. On a 46 m footing at 500 mm you'd need 92 stirrups. The spacing is a design value, so take it from your drawings, not from whichever example you happened to read first.

Cover and hooks set the stirrup length

A stirrup sits inside the concrete cover, so its sides are the footing width and depth minus twice the cover. That cover is not a free choice. It protects the steel from corrosion. Russia's concrete code calls for 40 mm in foundations cast on a concrete blinding layer, and engineering references give 70 mm without one. US guides commonly use 3 inches for concrete cast against earth.

One widely read supplier example works the default footing with only 25 mm of cover. That gives a stirrup perimeter of 1.8 m. At the code's 40 mm the perimeter is 1.68 m. More cover means a slightly shorter stirrup, so the lower cover doesn't save steel. It just puts the bars closer to the soil.

Then come the hooks. A closed stirrup needs two hooked ends to anchor it. A common shortcut from bar-bending practice (IS 2502) adds 24 bar diameters for the pair. For an 8 mm stirrup, that's 192 mm. With the hooks, each cut piece is 1.87 m. That is how 167 stirrups become 312.6 m. The supplier example instead took 300 m without hooks and added a rough 10%, which came to 330 m. The two figures are close, but only one of them shows you where the steel goes. Bending shops may use a slightly different hook convention, so check their cutting list if they make your stirrups.

Laps and corners: why 40d isn't a rule

Bars come in fixed stock lengths, so a 50 m footing needs lap splices. Corners and T-junctions need extra L-shaped bars too. Guides often state the lap as a flat "40 bar diameters, the code standard". Others say 30, 40 or 50, or 40 to 60.

In design codes, the lap length is calculated. It depends on the concrete class, the steel grade, bond conditions and how many bars are spliced at the same point. One engineering reference based on the Russian code gives at least 46 diameters for grade A500C bar in class B30 concrete when no more than half the bars are spliced in one section. When all of them are, it gives at least 76. A flat 40d can be too short.

That's why a material estimate covers laps, corner bars and offcuts with one percentage, as supplier worked examples do. They typically add 5 to 10%. In the example, 10% adds 20 m to the 200 m of longitudinal bar. If your drawings give a longer lap, or the plan has many corners and junctions, raise the percentage rather than guessing joint by joint.

From metres to kilograms and stock bars

Rebar is sold by weight, so the last step uses a mass-per-metre table. The values come from steel density, and they're the same in the Russian standard and a European B500B mill catalogue. One metre of 12 mm bar is 0.888 kg, and one metre of 8 mm is 0.395 kg. Real bars can differ from nominal mass by a few percent. The Russian standard allows roughly 4 to 8% depending on diameter, so a tonne of steel won't buy exactly the number of metres your estimate says.

To count stock bars, divide each total length by the bar length and round up. At 11.7 m bars, the example needs 19 bars for the longitudinal steel and 27 for the stirrups. That's a count with no cutting plan. If you work out how to cut each bar with less waste, you may need fewer.

These sizes are metric (10 to 25 mm longitudinal, 6 to 12 mm stirrups). US #4 or #5 bars have different diameters and weights, so they need their own table.

What the calculation can't decide for you

Everything above lays out a reinforcement scheme you already have: bar count, diameter, stirrup size and spacing. It doesn't tell you whether that scheme is strong enough. Some guides say things like "use #5 for a two-story house". Those are rules of thumb, and they don't agree with each other. Bar size and count depend on soil bearing capacity, wall loads and your local code. A foundation is a load-bearing structure, so that decision belongs to a structural engineer.

Once you have the scheme, the rebar calculator below runs the takeoff. Enter:

• the total strip length, outer perimeter plus interior walls;
• the section width and depth, and the cover your code requires;
• the bar count and diameter, stirrup diameter and spacing from the drawings;
• a lap and corner allowance, and the stock bar length your supplier sells.

It returns metres and kilograms for each bar size, the stirrup count and cut length, and the number of stock bars to order. For the concrete in the same footing, use the materials calculator.