Rebar Quantity Calculator
Calculate rebar quantity step by step: Bars each way = floor(clear span ÷ spacing) + 1.
In short
Calculate rebar quantity step by step: Bars each way = floor(clear span ÷ spacing) + 1.
Formula: Bars each way = floor(clear span ÷ spacing) + 1. Length = Σ bars × bar length × layers × (1 + waste). Mass = length × d² ÷ 162.2 kg/m.
What this calculator does
Calculate rebar quantity step by step: Bars each way = floor(clear span ÷ spacing) + 1. Worked example, questions and limitations included.
You enter slab length, slab width, bar spacing (centre to centre) and edge cover and a few other details, and the rebar quantity calculator shows the answer along with the working, so you can check it or try other numbers.
The page shows the formula, a numeric worked example, and the assumptions that affect this construction calculation.
Inputs and what they mean
- Slab length
- — number value.
- Slab width
- — number value.
- Bar spacing (centre to centre)
- — number value.
- Edge cover
- — number value.
- Bar diameter
- — number value.
- Layers (mats)
- — number value.
- Waste allowance
- — Enter your own allowance.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
d²/162.2 is the steel mass per metre for a round bar (density 7,850 kg/m³), d in mm.
Laps, hooks and chairs are not included — add them through the waste allowance or separately.
Cover is removed from both edges before counting bars.
Bars each way = floor(clear span ÷ spacing) + 1. Length = Σ bars × bar length × layers × (1 + waste). Mass = length × d² ÷ 162.2 kg/m.
Inputs used: Slab length, Slab width, Bar spacing (centre to centre), Edge cover, Bar diameter, Layers (mats), Waste allowance.
Worked example
10 × 6 m slab, 12 mm bars at 200 mm
- Clear: 9.9 × 5.9 m.
- Lengthwise bars: floor(5.9/0.2)+1 = 30 × 9.9 m = 297 m.
- Widthwise bars: floor(9.9/0.2)+1 = 50 × 5.9 m = 295 m.
- 592 m × 1.05 = 621.6 m; × 0.888 kg/m ≈ 552 kg.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Reference price tables
No reference prices are configured. Enter your own prices and rates — this site never assumes them.
Limitations and assumptions
- Rectangular slab, uniform spacing.
- No laps, hooks, chairs or openings.
- Quantities only — engineering design sets bar size and spacing.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this rebar quantity calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Reference: OpenStax (Rice University)
Last reviewed:
Common questions
Are lap splices included?
No. Where bars are longer than stock lengths, laps add material; include them in the waste allowance or calculate them from your drawings.
Where does d²/162.2 come from?
It is the cross-sectional area of a round bar times steel density 7,850 kg/m³, simplified for d in millimetres.
Why is waste not preset?
Waste depends on stock lengths, cutting plan and site practice, so you set it.
How do I use the Rebar Quantity Calculator?
Enter the required values for Slab length, Slab width, Bar spacing (centre to centre), Edge cover, Bar diameter, and the other fields shown. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Rebar Quantity Calculator use?
Bars each way = floor(clear span ÷ spacing) + 1. Length = Σ bars × bar length × layers × (1 + waste). Mass = length × d² ÷ 162.2 kg/m. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Rebar Quantity Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
Sources
Method based on OpenStax (Rice University).
Cite this page
Free Calculator. (2026, November 3). Rebar Quantity Calculator. https://freecalculator.tech/tools/rebar-quantity-calculator
"Rebar Quantity Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/rebar-quantity-calculator.
Rebar Quantity Calculator — Free Calculator, https://freecalculator.tech/tools/rebar-quantity-calculator
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