Steel Rebar Calculator

What is rebar?

Reinforced concrete works because two materials cover each other's weaknesses. Concrete is strong in compression but cracks under tension. Rebar — steel bars, usually with ridges (deformations) that grip the concrete — carries the tensile load. Together they act as one material: the concrete protects the steel from fire and corrosion, and the steel stops the concrete from cracking apart under bending, shrinkage and loads.

Bars are graded by yield strength — Fe415 and Fe500 are common — and sized by diameter. US sizes #3 to #8 (3/8 in to 1 in) cover most house slabs, footings and walls. Rebar goes wherever concrete is pulled or bent: the bottom of slabs, the sides of footings, the faces of walls, and inside columns and beams.

In corrosive settings, epoxy-coated or stainless bars are sometimes used, because ordinary steel rusts once water reaches it through cracks.

Rebar sizes and weight

The size number is the diameter in eighths of an inch: a #4 bar is 4/8 in, or 1/2 in, across. Weight per foot grows with the square of the diameter, so doubling the diameter quadruples the weight. That is why picking the right size matters more than adding extra bars.

Metric weight comes from the formula kg/m = d² ÷ 162.28 where d is the diameter in mm. US weight is kg/m ÷ 1.488. Sizes #3 to #8 cover residential work; heavier commercial work uses #9 to #11 bars, which this calculator does not include.

Metric bars are sized directly in millimetres: 10, 12, 16, 20 and 25 mm are the common sizes, weighing 0.616, 0.887, 1.578, 2.466 and 3.854 kg/m by the same d² ÷ 162.28 formula. A #4 US bar (12.7 mm) is close to a 12 mm metric bar but not identical, so do not mix the two in one cage.

Every bar carries mill marks rolled into the steel: the producer's mark, the size number and the grade. Check them on delivery — a bundle marked #4 that measures 12 mm is a metric bar, and its weight per foot is lower than the table says. Reject bars that do not match the order.

How to calculate rebar quantity

Work in this order, and every number follows from the one before it:

1. Bars each way. Bars along the width = floor(width × 12 ÷ spacing) + 1. Bars along the length = floor(length × 12 ÷ spacing) + 1 — dimensions are in feet, spacing in inches, so multiply by 12 first. The +1 is the bar at the far end — the most common mistake is forgetting it.
2. Total bars. Add both directions, then multiply by the number of layers.
3. Linear feet. Multiply each direction's bar count by the length it runs, then add and multiply by layers.
4. Weight. Linear feet × the bar's lb/ft from the table above.
5. Stock bars. Total linear feet ÷ stock length, rounded up.
6. Cost. Weight × your price per lb (or per kg).

Order with a cushion. Round stock bars up, keep two or three spare bars for miscuts, and add 5–10% to the weight for laps and waste. A short order stops the pour; a small surplus is cheap insurance, and leftover straight bars are usable on the next job. If your job has many laps or irregular shapes, use 10% rather than 5%.

Spacing by application

Spacing is centre-to-centre distance between parallel bars. Closer spacing means more steel, less cracking and higher cost. These are typical values for residential work — the engineer's drawings and local code always govern:

If you are unsure between two spacings, take the closer one: the extra steel is cheap compared with a cracked slab.

Spacing and bar size trade against each other. Steel area per foot of width = bar area × 12 ÷ spacing. A #4 bar (0.20 in²) at 12 in gives 0.20 in² per foot; a #5 bar (0.31 in²) at 18 in gives 0.21 in² per foot — nearly the same steel, fewer bars to tie. That is why engineers sometimes specify bigger bars at wider spacing.

Maximum spacing is also limited: many codes cap slab bar spacing at 18 in, because wider spacing lets cracks open between bars. If your calculation suggests 24 in, the code — not the calculator — wins. For two-way slabs, apply the spacing in both directions; a slab is not done when one direction is spaced and the other is forgotten.

Lap splices and stock length

Bars come in fixed stock lengths — 20 ft and 40 ft in the US, 6 m and 12 m metric. Any run longer than one stick needs a lap splice: two bars overlapped so force transfers from one to the next through the concrete.

The rule of thumb is a lap of 40 × bar diameter: a #4 bar laps 40 × 0.5 in = 20 in, a #5 laps 25 in, a #8 laps 40 in. Laps add length and weight, so add lap length to your linear feet before converting to weight and cost. Stagger laps so they do not all land in one section, and prefer 40 ft sticks where transport and handling allow — fewer laps, less waste.

Compression laps can be shorter than tension laps, because the concrete helps carry compression, but 40 diameters is the safe default when the drawings do not say otherwise. Stagger laps so no section has all its bars spliced at the same point — a section full of laps is a weak section. Tie every lap with at least two ties so it cannot open during the pour.

Lap weight adds up: ten #5 laps at 25 in each add 250 in, or 20.8 ft, which is 21.7 lb of steel. On a house with dozens of laps, ignoring them understates the order by a noticeable amount.

Cover, chairs and tie wire

Cover is the concrete between the bar and the surface. It protects the steel from water and rust: typically 1.5 in for formed surfaces and 3 in where concrete is cast against earth. Bars sitting on the ground with no cover rust, expand and crack the slab from inside.

Chairs hold the bars at the right height while concrete is poured. Space them on about a 4 ft grid: chairs per layer = (length ÷ 4 + 1) × (width ÷ 4 + 1). For a 20 × 15 ft slab that is 6 × 5 = 30 chairs per layer.

Tie wire fastens bars where they cross so the cage does not move during the pour. Tying every intersection is strongest; for slabs, every other intersection is common practice. Estimate ties from the intersection count and allow about 16 in of wire per tie plus waste — wire weight is small next to bar weight, but running out mid-pour is not.

Columns and beams usually need about 1.5 in of cover, while slabs on grade need more where they meet soil. Too little cover is the common failure: rust stains and spalling show within a few years. Too much cover is also wrong, because it pushes the steel away from the face that needs it and reduces the beam or slab's effective depth. Cover blocks, plastic chairs or concrete dobies hold the cage in place; never use wood offcuts or broken brick, which rot or crush.

Bar bending and cut list

Cut length is not the same as straight length. Hooks add length and bends consume it, so a cut list has to account for both:

• Standard 90° hook extension: 12 × bar diameter.
• 90° bend deduction: about 2 × bar diameter.
• Cut length = straight length + hook extensions − bend deductions.

Example: a 10 ft straight #4 bar with one 90° hook. Hook extension: 12 × 0.5 in = 6 in. Bend deduction: 2 × 0.5 in = 1 in. Cut length: 10 ft + 6 in − 1 in = 10 ft 5 in. A proper cut list has one row per distinct bar — mark, size, quantity, straight length, bends, cut length — plus about 5% spare for bending mistakes.

Hooks come in three standard forms. A 90° hook extends 12 diameters past the bend. A 135° seismic hook, used where earthquakes matter, extends 6 diameters. A 180° hook extends 4 diameters, but never less than 2.5 in. The bend itself is measured to the outside of the bar, which is why the deduction exists.

Stirrup example: a 12 × 18 in rectangular stirrup in #4 bar. Straight perimeter: 2 × (12 + 18) = 60 in. Two 135° hooks: 2 × 6 × 0.5 in = 6 in. Four 90° corner bends: 4 × 2 × 0.5 in = 4 in deducted. Cut length: 60 + 6 − 4 = 62 in, or 5 ft 2 in. Make one, check it against the drawing, then cut the rest.

Bend bars cold, never heated, and never re-bend a bar that has already been bent — re-bending cracks the steel at the bend. Bars #6 and larger are usually bent in the shop, where the bender and the measurements are exact.

Metric and per-kg pricing

Metric stock comes in 6 m and 12 m bars, and metric weights come from the kg/m column of the sizes table. Pricing follows the same pattern as the calculator: cost = weight × price. The only trap is the unit.

Price per tonne = 1,000 × price per kg. And a metric tonne (1,000 kg = 2,204.6 lb) is not a US short ton (2,000 lb) — a price quoted per short ton looks about 10% cheaper than the same price per tonne. Check the unit before comparing supplier quotes. Example: the worked slab above weighs 192.4 kg, so at $1.87/kg it costs about $360 — the same $360.55 the calculator gives at $0.85/lb.

Converting a US design to metric means matching steel area, not bar number. A #4 bar has 0.20 in² (129 mm²) of steel. A 12 mm bar has 113 mm² — slightly less — while a 16 mm bar has 201 mm², well more. The engineer picks the nearest size that does not reduce the steel area.

Metric lengths change the stock count too: a 12 m stick replaces a 40 ft stick almost exactly (39.4 ft), while a 6 m stick replaces 20 ft (19.7 ft). The calculator's stock count works the same way — total length divided by stick length, rounded up.

Two-layer mats and irregular shapes

Thicker slabs and driveways carrying real loads often need two layers: a bottom mat and a top mat, held apart by chairs tall enough to keep the top mat at its cover height. Two layers means double the bars in the calculator — set Layers to 2 — plus the taller chairs between them.

For irregular shapes, split the area into rectangles, calculate each rectangle on its own, and add the results. For a triangular corner, use the average bar length across that section rather than the longest bar. Always round up per section: rounding down in three sections quietly loses a full bar.

Chair height for the top mat comes from the slab thickness: thickness minus cover top and bottom minus two bar diameters. For an 8 in slab with 2 in cover each face and #5 bars: 8 − 2 − 2 − 2 × 0.625 = 2.75 in. Order chairs at that height, not at a guess — a top mat sitting an inch low loses a noticeable share of its strength.

Top mats also need chairs that will not tip when workers walk the cage — space them no wider than 4 ft.

L-shaped example: split an L into two rectangles, say 20 × 15 ft and 10 × 10 ft. Calculate each rectangle on its own at the same spacing and bar size, then add the bars, feet and weight. The corner where they meet is counted once, in the larger rectangle, so the bars there are not doubled.

Common mistakes

These mistakes cause most short orders and site arguments. Check your numbers against each one before ordering. Most of them cost nothing to avoid and a great deal to fix:

• Forgetting the +1 bar at each end. Bars equal spaces plus one: a 20 ft run at 12 in spacing has 20 spaces but 21 bars.
• Ignoring lap length. Laps add feet, weight and cost. Add lap length to your linear feet before converting to weight.
• Mixing clear spacing with centre-to-centre spacing. Clear spacing is the gap between bars; centre-to-centre spacing includes one diameter more.
• No waste allowance. Add 5–10% for laps, cuts and damage, or the last bars will come up short.
• Wrong cover. Too little cover lets bars rust and spall the concrete; too much cover weakens the section by moving steel from the face that needs it.
• Exact counts, no spares. Keep two or three spare bars for miscuts and bending mistakes.
• Mixing lb/ft and kg/m. Pick one unit system for the whole calculation and convert only at the end.
• Forgetting hooks and deductions. A cut list without hook extensions and bend deductions produces bars that do not fit.
• Ordering before drawings are final. A spacing change reworks every number, so confirm the drawings first.
• Storing bars on bare ground. Mud and standing water rust bars before the pour. Stack bars on timber, under cover.
• Not checking the delivery. Count bars and check sizes and grades against the order before the truck leaves — a wrong bundle found during the pour cannot be fixed.

Frequently asked questions