Size, spacing, cover — and the three numbers people all call "how much rebar"
← Back to the calculatorSame footing, three numbers, and only two of them can be ordered from. Net steel is what ends up in the concrete. Whole bars is what you can actually buy. Buy weight is what the counter charges for. The gap between 145 ft and 160 ft is 15 ft of offcut — a remainder, not a percentage.
This page also settles the two things every footing detail turns on: the 18 in spacing cap that no standard footing escapes, and the 3 in earth cover that decides how fast the steel corrodes.
Search "footing rebar" and most results give you a recipe: #4 or #5 bar, 12 in on centre, 3 in of cover, 40 × db lap. That recipe is not wrong, but it stops one step before the part that costs money. It never turns the spacing into a bar count, never checks the spacing against the code cap, and never separates the steel in the footing from the steel you have to buy.
So this page does the three things the recipe leaves out. It converts size and spacing into counts you can check against a drawing. It shows why the 18 in cap, not the thickness rule, governs every real footing. And it keeps net steel and buy weight in separate columns, because a footing is never ordered by the foot.
Four bar sizes cover almost every residential footing. The weight per foot is the ASTM A615 nominal value, and it is the number everything else is built on — a price, a count and an order weight all run through it.
| Bar | Diameter | lb per ft | Typical footing use |
|---|---|---|---|
| #3 | 3/8 in | 0.376 | Small pads, porch and step footings, curbs |
| #4 | 1/2 in | 0.668 | Standard residential continuous and pad footings |
| #5 | 5/8 in | 1.043 | Wider or heavier footings, mats, multi-story walls |
| #6 | 3/4 in | 1.502 | Commercial and heavy loads — engineered drawings |
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Which one you get is not a choice this page makes. It comes off the structural drawings and depends on load, footing size, soil and exposure. What matters here is that once the bar is named, its weight per foot — and so every count and cost below — follows from that one number.
Footing spacing is usually quoted as "12 in on centre", and that phrase hides an arithmetic step. A spacing is not a count until you divide the run by it. For the transverse bars, which cross the run, the count over a 40 ft footing (474 in clear, after 3 in of cover at each end) is:
| Spacing | Bars over a 40 ft run | What it is used for |
|---|---|---|
| 12 in | 40 | Standard residential footing |
| 16 in | 30 | Lighter load or narrow footing |
| 18 in | 27 | At the code ceiling (see next section) |
| 24 in | 20 | Only where the drawing allows it |
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The other direction — the longitudinal bars that run the length of the footing — is counted in lines across the width, not by spacing along the run. A narrow footing takes two or three; a wide one takes four or a mat. The typical minimum by width, with the drawings always governing:
| Footing width | Longitudinal lines (typical) | Cross-section |
|---|---|---|
| 12 in | 2 | One top, one bottom, or two in a single layer |
| 16–18 in | 3 | Two near the bottom, one near the top |
| 24 in | 4 | Two layers of two, or a four-bar cage |
| 30 in + | 4–6 or a mat | Wide footing, engineered |
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Two lines is the floor for a continuous footing, and that floor is set by design, not by the spacing rule. The spacing rule only sets a maximum gap; it never tells you the minimum number of bars, because at 12 in spacing a 12 in wide footing geometrically fits a single bar, and no real footing is built with one. That is why "what spacing" and "how many bars" are two different questions with two different answers.
Here is the detail most footing pages either skip or get half-right. The spacing rule crews apply from ACI 318 is: maximum spacing of the reinforcement is the lesser of 3 × the member thickness or 18 in. Written that way it sounds like thickness controls. Run the numbers and it does not — not on a footing.
| Footing thickness | 3 × thickness | Lesser of that or 18 in |
|---|---|---|
| 4 in | 12 in | 12 in — thickness governs |
| 6 in | 18 in | 18 in — the two are equal |
| 8 in | 24 in | 18 in — the cap governs |
| 10 in | 30 in | 18 in — the cap governs |
| 12 in | 36 in | 18 in — the cap governs |
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Look at where the crossover sits. The 3 × thickness term overtakes 18 in exactly at 6 in thick, and 6 in is the minimum footing thickness in the code table. So for every footing that is code-legal — 6 in, 8 in, 10 in, 12 in thick — the 3 × thickness figure is 18 in or more, which means:
Two caveats keep this honest. First, footings are designed members, and the code rule above is the slab spacing rule that practitioners carry onto footings; where the drawings give a closer spacing, the drawings win, and many engineers do. Second, a closer spacing is not free — going from 18 in to 12 in raises the transverse count by half, and every one of those bars is bought, cut, tied and inspected. The cap is a ceiling, not a target.
Cover is the concrete between the bar and the outside face, and on a footing the governing face is the one buried in soil. ACI 318 requires 3 in of clear cover for bars where concrete is cast against and permanently in contact with earth — which is exactly what a footing is.
| Condition | Minimum cover | Where you meet it |
|---|---|---|
| Cast against, in contact with earth | 3 in | Every footing side and bottom |
| Formed, exposed to earth or weather | 1.5 in | Formed footing face not touching soil |
| Formed, not exposed | 1.5 in | Interior faces |
| Slab interior, not exposed | 0.75 in | Not a footing — for contrast |
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Three inches is not a rounding-up of the slab number; it is four times it, and the reason is corrosion. Concrete protects steel by being alkaline and by keeping water and chloride away from it, and the soil side of a footing is where both arrive — groundwater, road salt, the damp that never dries. A bar with 3/4 in of cover on that face would start to corrode, the rust would expand, and the cover would spall off. Three inches of cover puts enough sound concrete between the steel and the ground that the chloride takes decades to reach it. It also has a physical consequence for the takeoff: the bars sit lower in the form, so the chairs are taller than a slab's, and the clear width and clear run are both 6 in smaller than the nominal dimensions.
Now the part the recipe leaves out. Take the same 16 in wide × 40 ft continuous footing, 3 × #4 longitudinal, #4 transverse at 16 in o.c., 3 in earth cover, and work it in the three columns.
| Net steel | Whole bars | Buy weight | |
|---|---|---|---|
| Linear feet | 145 ft | 8 × 20 = 160 ft | 160 ft |
| Weight, #4 | 96.9 lb | — | 106.9 lb |
| Order from it? | No — it is steel, not a product | Yes — count sticks | Yes — weight, what you pay |
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Read the gap. Net steel is 145 ft; you buy 160 ft because the transverse bars total 25 ft, and 25 ft of a bar that comes in 20 ft lengths is two sticks with 15 ft left over. That 15 ft cannot be shared with the longitudinal lines — they are already whole — so it is offcut, and every pound of it is invoiced. The order weight is 10.3% above the net steel, and that number is not a waste factor you typed in; it fell out of how 30 transverse bars of 10 in pack into 20 ft stock.
The three-column method is the same for every footing; what changes is how the two directions are named and counted.
floor(span ÷ spacing) + 1, each one as long as its span less two covers. There is no "longitudinal vs transverse"; both directions are structural.The stock-length decision — 20 ft vs 40 ft, and the laps each one forces — is the subject of its own page, because on a long footing run it moves the order weight more than the bar count does. That is the footing rebar calculator page. Once you know your buy weight, the cost per foot of footing page turns it into money.
It depends on the footing, but a 16 in wide by 40 ft long continuous footing with three #4 longitudinal bars and #4 transverse bars at 16 in on centre carries 145 ft of net steel, or 96.9 lb. You cannot buy 145 ft, though: on 20 ft stock the transverse bars round up to two sticks and the order becomes 8 whole bars, 160 ft and 106.9 lb of buy weight. Net steel is what is in the concrete; buy weight is what leaves the yard.
Most residential footings use #4 (1/2 in) or #5 (5/8 in) bar. A narrow continuous footing commonly takes two or three #4 longitudinal bars; a wider or heavier footing steps up to #5 or several lines. When the drawings call for #6 or larger you are in engineered territory and should be working from stamped plans. Bar size, count and spacing are the engineer's, not a rule from a website.
Twelve inches on centre is the common starting point for residential footings, tightened to 6 to 8 in under heavy loads. The ceiling comes from the spacing rule crews apply from ACI 318: maximum spacing is the lesser of 3 × the member thickness or 18 in. On every footing 6 in thick or more — which is every code-minimum footing — 3 × the thickness exceeds 18 in, so the 18 in cap is what actually sets the maximum. The drawings set the actual spacing.
Because a footing is cast against and permanently in contact with earth, and ACI 318 requires 3 in of clear cover where concrete touches the ground. That is the governing case for a footing and it is far more than the 3/4 in used inside a dry slab, because the soil side is where water and chloride reach the steel. Deeper cover means the bars sit lower and the chairs have to be taller.
A continuous wall footing works two ways for two reasons: longitudinal bars run the length of the run to carry bending, and transverse bars are spaced along it to distribute load and hold the cage together. A spread or pad footing under a column is reinforced in both directions as a mat, because it bends in both directions. The drawings give the direction and count; this page turns those into a takeoff.
Between about 2.5 and 5.5 ft of bar per foot of footing, depending on width and layout. A 12 in wide footing with two #4 longitudinal bars and #4 transverse at 12 in uses about 2.5 ft of bar per foot. A 16 in footing with three #4 and #4 at 16 in uses about 3.6 ft per foot. A 24 in footing with four #5 and #5 at 12 in uses 5.5 ft per foot. Multiply that by the bar's price per foot to get the cost per foot of footing.
Work the order in whole stock bars and the cut waste is already in it, because you are buying complete bars, not a length. On the 16 in by 40 ft example the 30 transverse bars add up to 25 ft, which rounds up to two 20 ft sticks and leaves 15 ft of offcut — that offcut is a remainder, not a percentage you can plan. Add only a small handling allowance for cutting error and damaged ends.