Rebar & Footing Calculator

What a footing size lookup means

Table R403.1(1) is a lookup, not a formula. The row is set by story count (1, 2, or 3) plus what sits between the footing and the floor — slab-on-grade, crawl space, or plus basement. A basement wall carries more load, so it needs a wider footing. The column is soil bearing in psf: R401.4.1 presumptive values run clay 1,500, sand 2,000, gravel 3,000. Softer soil spreads the load over more area, so clay drives a wider footing than gravel.

Four tables split the results by roof and snow load — 20, 30, 50, and 70 psf. The calculator assumes a 32-ft building width, 9-ft walls, 8-ft basement walls, a clear-span roof, and an interior bearing wall or beam at each floor; the footnote adjusts about 2″ of width and 1″ of thickness per 4 ft off that baseline. Width may be interpolated between soil columns, never extrapolated. The floor is 12″ × 6″ (R403.1.1); where the table runs out, you need an engineer.

What horizontal and vertical bars do

Concrete is strong in compression and weak in tension. A footing under a bearing wall acts like a beam — soil pushing up, wall load pushing down — so where soil is softer under part of the run, the horizontal bars carry the bending tension. They also tie the pour into one continuous member.

Vertical bars stick up out of the footing into the stem wall and tie the wall to the footing against sliding, overturning, and lateral soil pressure. A turned-down slab footing may use one #4 top and one #4 bottom, or a single #5 or two #4s in the middle third (R403.1.3.3).

Is rebar required? It depends on your seismic category

R403.1.3 requires reinforcement in footings and stem walls only in Seismic Design Categories D0, D1, and D2. In SDC A, B, and C — most of the country — the IRC permits plain concrete footings. There is no steel mandate to point at.

Where it is required the minimums are modest: one #4 vertical at no more than 4 ft on center, hooked at least 14″ into the stem wall; one #4 horizontal within 12″ of the stem wall top; one #4 horizontal 3″ to 4″ from the footing bottom. Masonry stem walls are solid grouted. So the two-bar takeoff below is common practice and engineer spec, not an IRC mandate in SDC A/B/C. Confirm your seismic category with the local AHJ.

Lap splices, plainly

Stock rebar comes in 20-ft sticks, so longer runs get spliced. A lap is two bars overlapped so force passes from one to the other through bond with the concrete, and the overlap has to be long enough to develop the bar, which is why lap length comes from a table.

The calculator uses the ACI 318-19 Class B tension lap for Grade 60 bar, simplified method: 25″ for #4 and 31″ for #5 at 4,000 psi, 29″ and 36″ at 3,000 psi, 23″ and 28″ at 5,000 psi. Stronger concrete grips harder, so the lap shortens. Noncontact laps are allowed a gap no larger than the smaller of one-fifth of the lap or 6″ (R403.1.3.5.4).

Cover: cast against earth, formed, exposed

R403.1.3.5.3 sets cover three ways: 3″ cast against earth, including the bottom of a footing; 1.5″ for #5 and smaller in removable forms exposed to weather and 2″ for #6 and larger; and 0.75″ where the concrete is not exposed. On a 6″ footing that 3″ means chairs tall enough to lift the bars clear of the mud.

Bar sizes and weights

The bar number is eighths of an inch of diameter, so a #4 is ½″. ASTM A615 nominal weight is 2.67 × the diameter in inches, squared: #3 (⅜″) 0.376 lb/ft, #4 (½″) 0.668, #5 (⅝″) 1.043, #6 (¾″) 1.502. A 20-ft #4 stick weighs about 13.4 lb. The rebar size chart carries the full #3–#18 table — every diameter, area and unit weight, with a take-off to pounds and tons.

Worked example — 30-ft run, 2 × #4, 4,000 psi

Two #4 bars running 30 ft in 4,000 psi concrete:

• Sticks per bar: 30 ÷ 20 = 1.5 → 2 sticks, 1 splice per bar
• Lap allowance: 25″ = 2.08 ft
• Length one bar covers: 30 + 2.08 = 32.08 ft
• Total length: 2 × 32.08 = 64.17 ft
• Weight: 64.17 × 0.668 = 42.9 lb
• Sticks to buy: 2 × 2 = 4 (about $48 at $12 a stick)

The lap, not the footing, is where the waste lives: cut the same run to 20 ft and it takes 40 ft with no splice, about 26.7 lb.

The concrete around the steel

R402.2 sets the minimums: 2,500 psi for foundations and interior slabs; 3,000 psi for weather-exposed walls; 3,000 psi for porch and garage slabs in moderate exposure, 3,500 psi in severe; 5–7% air entrainment in moderate or severe freeze-thaw. On slabs, R506.1 sets 3.5″ minimum thickness and R506.2.4 puts reinforcement in the middle-to-upper third if it is used. The IRC does not require slab steel at all — mesh or bar in a residential slab is spec-driven.

Common mistakes

• Assuming rebar is mandatory everywhere. SDC A, B, and C permit plain concrete; reinforcement is required only in D0–D2 (R403.1.3).
• Wire mesh laid on the ground. Steel on the subgrade does nothing — R506.2.4 keeps slab reinforcement in the middle-to-upper third.
• Too little cover. Concrete cast against earth needs 3″ (R403.1.3.5.3); bars sitting on the mud rust from underneath.
• Laps that are too short. 25″ for #4 at 4,000 psi is a minimum, not a target (ACI 318-19 Class B).
• Mixing bar sizes in one run. Laps are sized per bar, so a #4 spliced to a #5 has no single lap length — use the longer figure.
• Buying by bar count instead of weight. Bar count says how many runs; stick count and weight say what to order.

Rebar and footing FAQ

Related pages

• Concrete calculator — volume, bags, and cost
• Deck footing calculator — IRC Table R507.3.1
• Guide: how to calculate concrete for a slab
• Rebar size chart — #3 to #18, diameter, area and weight