CalculationTime

Rebar Calculator

Estimate rebar pieces, total bar length and material cost for a rectangular slab or pad using spacing, edge cover, lap allowance and a printable job note.

Live math canvas

Your numbers, formula and explanation together

Rebar Calculator: 63 linear feet with bar counts and allowance. Rebar Calculator uses the declared inputs to produce a transparent default result. Slab length: 20 ft; Slab width: 12 ft; Rebar spacing: 18 in on centre; Edge cover: 3 in; primary comparison: 20 and 12.

Formula applied

The exact method behind this answer

CalculationTime keeps the method visible so the number can be checked instead of blindly trusted.

Clear length = slab length − 2 × cover. Clear width = slab width − 2 × cover. Bars across width = floor(clear width ÷ spacing) + 1. Bars across length = floor(clear length ÷ spacing) + 1. Total bar length = bars across width × clear length + bars across length × clear width. Order length = total bar length × (1 + lap/waste percent ÷ 100).
  1. Apply the formulaClear length = slab length − 2 × cover. Clear width = slab width − 2 × cover. Bars across width = floor(clear width ÷ spacing) + 1. Bars across length = floor(clear length ÷ spacing) + 1. Total bar length = bars across width × clear length + bars across length × clear width. Order length = total bar length × (1 + lap/waste percent ÷ 100).63 linear feet with bar counts and allowanceRebar Calculator uses the declared inputs to produce a transparent default result. Slab length: 20 ft; Slab width: 12 ft; Rebar spacing: 18 in on centre; Edge cover: 3 in; primary comparison: 20 and 12.

Your live breakdown

Current inputs in the calculation

These values come from the controls above and update when the calculator changes.

Slab length
20 ft
The longer direction of the slab, footing pad or rectangular concrete area.
Slab width
12 ft
The shorter direction of the slab or pad.
Rebar spacing
18 in on centre
Typical planning spacing between parallel bars. Engineering drawings override this calculator.
Edge cover
3 in
Distance from slab edge to the first bar, used on both sides before spacing bars across the clear span.
Lap and waste allowance
10 %
Planning allowance for laps, cuts, bends, breakage and site handling.
Optional bar price
0 $/ft
Optional estimating price for a quick material-cost note. Leave as 0 if not needed.

Resulting answer

63 linear feet with bar counts and allowance

Rebar Calculator uses the declared inputs to produce a transparent default result. Slab length: 20 ft; Slab width: 12 ft; Rebar spacing: 18 in on centre; Edge cover: 3 in; primary comparison: 20 and 12.

Answer
63 linear feet with bar counts and allowance
Live support
Rebar Calculator uses the declared inputs to produce a transparent default result. Slab length: 20 ft; Slab width: 12 ft; Rebar spacing: 18 in on centre; Edge cover: 3 in; primary comparison: 20 and 12.

Assumptions used

What this answer assumes

Best for early material takeoffs, slab quote checks, owner-builder notes, classroom grid examples and job files where rebar assumptions need to be visible beside the quantity.

  • The calculator estimates a simple rectangular two-way rebar grid, not a certified structural design.
  • Spacing and edge cover are planning inputs; engineering drawings, local code and site inspection override any generic calculator result.
  • Bar length is measured inside the edge-cover line. It does not automatically add hooks, chairs, ties, dowels, mesh, bent bars or footing-specific reinforcement.
  • The lap/waste percentage is a procurement allowance for cuts, laps and handling; exact lap lengths depend on bar size, concrete strength, code and design details.

Master’s Tip

How to use the result well

Master’s Tip: never let a rebar calculator replace the drawing. Use it to catch quantity mistakes, prepare a quote note or check a takeoff; use the engineer’s schedule for bar size, spacing, cover, laps, chairs and inspection requirements.

Printable record

What belongs in the saved calculation

Save the inputs, result, formula, assumptions, page URL and date together so the calculation can be reviewed later.

Slab length
20 ft
The longer direction of the slab, footing pad or rectangular concrete area.
Slab width
12 ft
The shorter direction of the slab or pad.
Rebar spacing
18 in on centre
Typical planning spacing between parallel bars. Engineering drawings override this calculator.
Edge cover
3 in
Distance from slab edge to the first bar, used on both sides before spacing bars across the clear span.

Embeddable calculator

Embed this calculator

Copy a clean iframe version with the required CalculationTime attribution link built in.

Formula

Clear length = slab length − 2 × cover. Clear width = slab width − 2 × cover. Bars across width = floor(clear width ÷ spacing) + 1. Bars across length = floor(clear length ÷ spacing) + 1. Total bar length = bars across width × clear length + bars across length × clear width. Order length = total bar length × (1 + lap/waste percent ÷ 100).

Worked example

For a 20 ft by 12 ft slab with 18 in spacing and 3 in cover, the clear length is 19.5 ft and the clear width is 11.5 ft. Bars across the width: floor(11.5 ÷ 1.5) + 1 = 8. Bars across the length: floor(19.5 ÷ 1.5) + 1 = 14. Total before allowance is 8 × 19.5 + 14 × 11.5 = 317 ft. With 10% allowance, order about 348.7 linear ft.

Professional note

Master’s Tip: never let a rebar calculator replace the drawing. Use it to catch quantity mistakes, prepare a quote note or check a takeoff; use the engineer’s schedule for bar size, spacing, cover, laps, chairs and inspection requirements.

Regional and unit assumptions

Standard or basis: rectangular grid takeoff arithmetic using feet, inches, on-centre spacing and edge cover. This is an estimating aid only and does not certify ACI, Eurocode, Australian Standard or local building-code compliance.

Assumptions and limitations

Methodology & Accuracy

How this calculator is checked

CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.

Formula used

Clear length = slab length − 2 × cover. Clear width = slab width − 2 × cover. Bars across width = floor(clear width ÷ spacing) + 1. Bars across length = floor(clear length ÷ spacing) + 1. Total bar length = bars across width × clear length + bars across length × clear width. Order length = total bar length × (1 + lap/waste percent ÷ 100).

Standard or basis

Standard or basis: rectangular grid takeoff arithmetic using feet, inches, on-centre spacing and edge cover. This is an estimating aid only and does not certify ACI, Eurocode, Australian Standard or local building-code compliance.

Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.

Master's Tip

Master’s Tip: never let a rebar calculator replace the drawing. Use it to catch quantity mistakes, prepare a quote note or check a takeoff; use the engineer’s schedule for bar size, spacing, cover, laps, chairs and inspection requirements.

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These cards are resolved from calculator data, category context and live-route status so the page links to useful next steps without hand-built link clutter.

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Questions

How do I calculate rebar for a slab?

Subtract edge cover from both sides to get the clear length and width, divide each clear span by the spacing, add one bar for the starting line, then multiply each bar count by the bar run length.

What spacing should I use for rebar?

Use the spacing shown on the engineering drawing or local requirement. This calculator accepts any spacing for estimating, but it does not decide structural spacing for you.

Does the rebar calculator include lap splices?

It includes a percentage allowance for laps and waste, but it does not calculate code-specific lap splice length. Exact lap rules depend on bar size, concrete strength, development length and design details.

Why does edge cover matter?

Edge cover keeps steel away from the outside face of the concrete. In a takeoff, cover also reduces the clear grid length used for each bar run.

What should I print for a rebar quote note?

Print slab length, width, spacing, cover, bar counts both ways, total linear feet, lap/waste allowance, formula, assumptions, date and job notes so the takeoff can be checked before ordering.

Calculation note

Rebar takeoffs are grid calculations with a safety boundary. The arithmetic is simple, but the consequences are not: spacing, cover, lap, chairing and bar size belong to the structural design. A good printable estimate therefore keeps the calculator result separate from the professional design assumptions.

A rebar grid is counted in two directions

Bars running one way are spaced across the other dimension. That is why a rectangular slab needs two counts: bars that run lengthwise and bars that run widthwise.

Cover is both practical and structural

Concrete cover helps protect reinforcement from exposure and corrosion. For estimating, the same cover distance also defines the clear bar run inside the slab edge.

Printed takeoffs reduce ordering mistakes

A job note with dimensions, spacing, cover, allowance and bar counts is easier to compare with drawings, supplier quotes and site changes than a single unexplained total.

Sources and further readingAmerican Concrete Institute — concrete resources and standardsPortland Cement Association — reinforced concrete basics