# Rebar Grid Calculator (Imperial Bar Sizes)

US bar size and spacing to counts, total length and pounds.

- Page: https://jianzhujisuan.com/en/rebar-grid
- Category: Rebar (https://jianzhujisuan.com/en/c/rebar)
- Chinese version: https://jianzhujisuan.com/zh/rebar-grid.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Length 20 ft, Width 10 ft, Bar size #4 (0.668 lb/ft), Spacing 12 in o.c., Layers Double layer, Lap allowance 10%): **Total rebar length 946 ft**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Length | number | 20 | ft |  |
| Width | number | 10 | ft |  |
| Bar size | select | #4 (0.668 lb/ft) | — |  |
| Spacing | select | 12 in o.c. | — |  |
| Layers | select | Double layer | — |  |
| Lap allowance | number | 10 | % |  |

## Formula

- **count = floor(span(ft) × 12 ÷ spacing(in)) + 1**: the +1 is the edge bar
- **net length = Σ(count × span) for both directions**: 
- **total = net × layers × (1 + lap%)**: 
- **weight = total × unit weight**: #3 0.376 / #4 0.668 / #5 1.043 / #6 1.502 lb/ft

## Worked example

A 20 ft x 10 ft slab with #4 at 12 in o.c., two layers, 10% laps: 10 × 12 / 12 + 1 = 11 bars one way and 20 × 12 / 12 + 1 = 21 the other, so 11 × 20 + 21 × 10 = 430 ft per layer. Two layers is 860 ft, 946 ft with laps, times 0.668 = 632 lb, about 0.32 short tons.

## Questions

### What size is #4 rebar?

Bar size times 1/8 in gives nominal diameter, so #4 is 4/8 = 1/2 in (12.7 mm), #3 is 3/8 in, #5 is 5/8 in and #6 is 3/4 in. That rule is nominal - actual rolled size varies slightly - so use nominal unit weight for quantities. Metric equivalents: #4 is close to 12 mm, #5 to 16 mm, #6 to 20 mm.

### Why is there a plus one on the bar count?

Because spacing measures centre to centre, and both edges need a bar. Over 10 ft at 12 in: bars fall at 0, 1, 2 ... 10 ft, which is 11 bars, not 10. On a large slab that one bar is lost in the rounding, but on a 3 ft wide strip footing missing it is 25% short.

### What lap percentage should I use?

For ordering, 10 to 15% of total length is the usual allowance. Proper calculation runs bar size, concrete strength, whether it is in tension, cover and spacing factors, and typically lands between 30 and 50 bar diameters. Also note more laps is not better - where more than 50% of bars are spliced at one section, lap length must increase.

## Related tools

- [Rebar Weight Calculator](https://jianzhujisuan.com/en/rebar-weight): Pick a bar diameter, enter length and quantity to get weight per metre, per bar and total tonnage.
- [Rebar Anchorage Length Calculator](https://jianzhujisuan.com/en/rebar-anchorage): Basic and seismic anchorage length from bar grade, diameter and concrete class.
- [Rebar Lap Length Calculator](https://jianzhujisuan.com/en/rebar-lap): Lap length ll / llE by splice percentage, plus the permitted number of splices.
- [Rebar Cut Length Calculator](https://jianzhujisuan.com/en/rebar-cut-length): Cut length and bar weight with standard hook allowances.
- [Rebar Spacing and Bar Count Calculator](https://jianzhujisuan.com/en/rebar-spacing): Bars and actual spacing from run length and design spacing, with total weight.
- [Stirrup Length and Count Calculator](https://jianzhujisuan.com/en/rebar-stirrup): Stirrup length, count and weight with the 11.9d seismic hook allowance.

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