Civil & Structural Engineering

Rebar Weight Calculator

Estimate reinforcing bar weight from rebar size, length, quantity or spacing, and waste allowance.

Formula Weight = unit weight × L × N × (1 + waste%)Reviewed Sep 8, 2026

Rebar weight is usually needed before ordering steel for slabs, footings, walls, beams, and small concrete jobs. This calculator converts standard rebar size, cut length, direct quantity or spacing layout, and optional waste allowance into total weight in kilograms and pounds. It is meant for material estimating and takeoff checks, not for structural reinforcement design.

Calculation Bench

Quantity Method

Bar Size

Nominal Rebar Size

Nominal diameter 15.875 mm · 1.043 lb/ft

Length & Count

01Length per Bar

L · Cut length of one bar

02Quantity

N · Number of bars

Allowance & Pricing

03Waste Allowance

% · Optional extra for laps, cuts, and waste

04Price per Kilogram

$/kg · Leave blank to skip cost estimate

Solution

Select a rebar size and enter length/count to estimate total reinforcing steel weight.

Weight = unit weight × L × N × (1 + waste%)

Formula Sheet

w_m = \dfrac{\pi d^2 \rho}{4}

W = w_m \times L \times N \times \left(1 + \dfrac{p}{100}\right)

N = \left\lceil\dfrac{B - 2c}{s}\right\rceil + 1

WTotal Rebar Weight
w_mUnit Weight
dNominal Diameter
ρSteel Density
LLength per Bar
NBar Count
BLayout Width
sSpacing
cEdge Cover
pWaste Allowance

Variables & Units

Symbol	Variable	Description	Common Units
W	Total Rebar Weight	Final reinforcing steel weight after allowance.	kg, lb, ton
w_m	Unit Weight	Nominal weight per metre for the selected bar size.	kg/m, lb/ft
d	Nominal Diameter	Plain-round equivalent diameter of the rebar.	mm, in
ρ	Steel Density	Density used for custom diameter weight calculations; default is 7,850 kg/m³.
L	Length per Bar	Cut length of one straight bar.	m, ft
N	Bar Count	Number of bars, either entered directly or calculated from spacing.
B	Layout Width	Overall width across the spaced bar set.	m, ft
s	Spacing	Maximum center-to-center spacing between bars.	mm, in
c	Edge Cover	Clear offset at both edges before spacing bars.	mm, in
p	Waste Allowance	Percentage added for laps, hooks, cuts, and field waste.

How to Use This Calculator

01Select the quantity method: Known Quantity when you already know the number of bars, or Spacing Layout when bars are placed at a regular spacing across a width.
02Choose a standard rebar size such as #4 / 13M or #5 / 16M, or use Custom Diameter for nonstandard reinforcing steel.
03Enter the cut length of one bar. For spacing layouts, enter the layout width, spacing, and optional edge cover.
04Add a waste allowance if you want extra steel for laps, hooks, cutting loss, field trimming, and small takeoff errors.
05If your drawing or bar schedule already includes lap splices and hooks in the bar length, do not add them again as waste unless you intentionally want an ordering buffer.
06Optionally enter a price per kilogram to estimate material cost from the calculated order weight.

How the Formula Works

The calculator first finds unit weight per metre. Standard # bar sizes use nominal table weights; a custom diameter uses the plain-round equivalent formula: unit weight = π × d² / 4 × ρ, where d is diameter and ρ is steel density.

For a direct quantity estimate, total raw length is length per bar multiplied by bar count. For a spacing layout, the clear layout width is the overall width minus cover at both edges, and bar count is rounded up so the actual spacing does not exceed the requested spacing.

The waste allowance is applied to length before weight is calculated. A 5% allowance means the order length is 1.05 times the raw cut length; the same factor applies to weight.

The result is a material takeoff estimate. Bar bends, hooks, lap splice rules, development length, structural spacing limits, grade, and code compliance still need to come from the project drawings or a qualified designer.

Worked Example 01

Ten #5 bars, 6 m each, with 5% allowance

Known

Bar size: #5 / 16M
Unit weight: about 1.552 kg/m
Length per bar: 6 m
Quantity: 10 bars
Waste allowance: 5%

Formula

Weight = unit weight × L × N × (1 + waste%)

Substitution

Order length = 6 × 10 × 1.05 = 63 m; Weight = 1.552 × 63

Result

≈97.8 kg

The raw cut length is 60 m. Adding 5% gives 63 m of order length, so the total #5 rebar weight is about 97.8 kg before supplier rounding.

Worked Example 02

#4 bars at 200 mm spacing across a 3 m slab strip

Known

Bar size: #4 / 13M
Layout width: 3 m
Edge cover: 50 mm at both edges
Spacing: 200 mm
Length per bar: 3 m

Formula

Bars = ceil((B − 2c) / s) + 1

Substitution

Bars = ceil((3.0 − 2×0.05) / 0.2) + 1 = 16 bars; raw length = 48 m

Result

≈47.7 kg before allowance

The 2.9 m clear width needs 16 bars so the actual spacing stays at or below the requested 200 mm. At about 0.994 kg/m, 48 m of #4 bar weighs about 47.7 kg.

Common ASTM Rebar Size Reference

Size	Nominal diameter	Nominal weight
#3 / 10M	9.5 mm	0.56 kg/m (0.376 lb/ft)
#4 / 13M	12.7 mm	0.994 kg/m (0.668 lb/ft)
#5 / 16M	15.9 mm	1.552 kg/m (1.043 lb/ft)
#6 / 19M	19.1 mm	2.235 kg/m (1.502 lb/ft)
#7 / 22M	22.2 mm	3.042 kg/m (2.044 lb/ft)
#8 / 25M	25.4 mm	3.973 kg/m (2.67 lb/ft)
#9 / 29M	28.7 mm	5.06 kg/m (3.4 lb/ft)
#10 / 32M	32.3 mm	6.404 kg/m (4.303 lb/ft)
#11 / 36M	35.8 mm	7.907 kg/m (5.313 lb/ft)
#14 / 43M	43.0 mm	11.38 kg/m (7.65 lb/ft)
#18 / 57M	57.3 mm	20.24 kg/m (13.6 lb/ft)

Applications

01Estimating rebar tonnage for slabs, footings, walls, grade beams, and small concrete pours
02Checking a contractor or supplier takeoff against a quick independent calculation
03Converting bar schedules into approximate shipping weight and material cost
04Planning extra allowance for field cutting, laps, hooks, and short offcuts

Assumptions

01Standard bar sizes use nominal table weights and nominal plain-round equivalent diameters, not measured rib-to-rib dimensions.
02Custom diameter uses 7,850 kg/m³ steel density unless the implementation is modified; this is the common carbon-steel estimate.
03Spacing mode assumes one straight bar line across a rectangular width, with equal maximum spacing and optional edge cover at both sides.
04Waste allowance is a takeoff buffer only. It does not calculate required lap length, hook length, bend deductions, or development length.

Where This Model Stops

01Does not design reinforcement area, bar spacing, cover, development length, hooks, or lap splices for code compliance.
02Does not optimize stock lengths or produce a bar bending schedule.
03Does not include tying wire, chairs, supports, couplers, epoxy/coating weight, or delivery packaging.
04Standard bar weights vary slightly by national standard and supplier. Use project specifications or supplier tables for final purchase orders.
05For critical procurement, confirm with project drawings, mill certificates, supplier weight tables, and local code requirements.

References

[1]
ASTM A615/A615M Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement
ASTM International
Primary specification family for common carbon-steel reinforcing bar designations, nominal dimensions, and nominal weights.
[2]
CRSI Ready Reference Mobile App
Concrete Reinforcing Steel Institute
CRSI describes its ready reference as including ASTM reinforcing bar sizes, diameters, areas, and weights.

Frequently Asked Questions

What is the formula for rebar weight?

For a custom round-bar estimate, unit weight per metre is π × d² × ρ / 4. For standard # sizes, use the nominal table weight per metre. Total weight is unit weight multiplied by total length, then adjusted by any waste allowance.

How do I calculate rebar quantity from spacing?

Subtract cover at both edges from the overall width, divide the clear width by spacing, round up, then add one bar. This keeps the actual spacing from exceeding the target spacing.

Is this the same as a structural rebar design calculator?

No. This estimates material weight only. Required bar size, spacing, cover, lap splices, hooks, and development lengths must come from engineering design or the project drawings.

Why are standard # sizes different from exact geometry?

Rebar tables use nominal plain-round equivalent dimensions and nominal weights. Deformed ribs make the measured shape irregular, so standard tables are the right basis for ordering estimates.

Should I add waste allowance?

Usually yes for procurement estimates. A small allowance helps cover laps, hooks, field cuts, offcuts, and counting errors, but the correct percentage depends on the project and bar schedule.

Does this include lap splices and hooks?

Only if they are included in the length you enter or in the waste allowance you choose. The calculator estimates weight; it does not calculate code-required lap splice, hook, or development lengths.

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