Rebar weight and quantity calculator

Introduction

Reinforcing bar is ordered by weight but placed by count and spacing. A #4 bar, half an inch across, weighs 0.668 lb per foot; a 12 mm metric bar weighs 0.888 kg per metre. A 20 × 20 ft slab with #4 bars at 12 in each way needs 40 bars, 780 ft of bar and about 520 lb of steel before laps.

This calculator gives the weight per length of US bars #3 to #18 and metric bars 6 to 50 mm, and counts the bars, total length and weight either from a number of pieces or from a slab or wall grid at a given spacing. It also gives the steel area per foot or metre of width, the figure a designer checks against the required reinforcement.

Formulas

Metric bar mass per metre (the d²/162 rule):

m' = 7 850 × π d² / 4 = 0.006165 d²    kg/m, d in mm

Bars in a grid, each way:

N = floor( (S − 2c) / s ) + 1,   each of length L − 2c
total length = Σ N × (L − 2c) × layers × (1 + allowance)

Steel area per width:

A_s = A_bar / s        in² per ft or mm² per m

• S — the span across which the bars are spaced; L — the length they run
• s — spacing; c — edge cover
• US bars: area and unit weight from ASTM A615 Table 1 (#4: 0.20 in², 0.668 lb/ft)

Assumptions and limitations

• US unit weights are the ASTM A615 nominal values. Metric bars use 7 850 kg/m³ on the nominal diameter.
• Grid bars run the full length less the cover at each end. Hooks, bends, laps and chairs go into the allowance percentage. A lap is typically 40 to 60 bar diameters.
• The bar size and spacing come from the structural design and the code (ACI 318, EN 1992-1-1). This page estimates quantities for ordering.
• The bar count rounds down the spaces and adds one bar, so the edge bars sit at the cover. Some detailers add an extra bar where the last space is short.

Worked example

A 20 × 20 ft slab has a single mat of #4 bars at 12 in each way, with 3 in of cover at the edges. How much steel is needed?

1. Bars across 20 ft: (240 − 2 × 3) / 12 = 19.5, so floor 19 + 1 = 20 bars.
2. Each bar is 240 − 6 = 234 in, or 19.5 ft long. The other direction is the same.
3. Total: 40 bars, 40 × 19.5 = 780 ft.
4. Weight: 780 × 0.668 = 521.0 lb.

With a 10 % allowance for laps and waste, order about 575 lb, which is 43 bars of 20 ft.

FAQ

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