Rebar Calculator
Calculate the bar count, total linear length, and weight of a two-way reinforcing steel grid for a rectangular concrete slab, from the slab size, bar size (#3-#10), spacing, clear cover, and a waste/lap allowance — with an option to double up for a top-and-bottom mat.
Input
US bar number with its soft-metric designation and nominal diameter.
% added to the total length for cutting waste and bar laps/overlaps.
Output
| Property | Value |
|---|---|
| No data yet | |
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/rebar-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"slabLength": "10",
"slabWidth": "10",
"dimensionUnit": "ft",
"barSize": "#4",
"spacing": "12",
"cover": "2",
"spacingUnit": "in",
"layers": "single",
"wastePercent": "10"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `rebar-calculator` tool (Rebar Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
Embed widget
<iframe
src="https://iotools.cloud/embed/rebar-calculator/"
width="100%" height="520" frameborder="0" scrolling="no" loading="lazy"
title="Rebar Calculator — iotools.cloud"
sandbox="allow-scripts allow-forms allow-same-origin allow-downloads allow-popups allow-popups-to-escape-sandbox"
allow="clipboard-write"
style="width:100%;border:1px solid #e5e7eb;border-radius:12px;overflow:hidden"></iframe>
<script src="https://iotools.cloud/embed.js" async></script>Drop this into your own page — free, no key required, just a link back.
| Cost per API/MCP call | From 5 credits |
|---|---|
| Need more credits? | View pricing |
Also available with
Guides
Work out how many rebar bars a rectangular concrete slab needs, in both directions of a two-way grid, plus the total linear length and weight of steel to order. Enter the slab size, bar size (#3-#10), on-center spacing, clear cover, and a waste/lap allowance, and the calculator returns the bar count and cut length for each direction, the total length before and after waste, and the total weight in both pounds and kilograms.
How to use it
- Enter the slab length and slab width, and pick the unit (feet or meters).
- Pick the bar size — #3 through #10, the sizes typically used in a slab mat (larger bars are for columns and beams, not slabs).
- Enter the bar spacing (on-center, used for both directions of the grid) and the clear cover from each edge, in inches or millimeters.
- Choose single layer for a bottom mat only, or two layers to double everything for a top-and-bottom mat.
- Set the waste/lap allowance — the percentage added to the total length for cutting waste and bar overlaps.
Results update as you type: the bar count and cut length for each direction, the total bar count, the total length before and after waste, and the total weight.
The math
Bars in one direction run the slab's length, spaced across its width; bars in the other run the width, spaced across the length. Both stop cover short of every edge:
- Count (per direction) = ⌊(span across − 2 × cover) / spacing⌋ + 1
- Cut length (per direction) = span along − 2 × cover
The two directions' totals are summed, doubled if you picked two layers, then increased by the waste percentage. Weight comes from the standard ASTM A615/A615M unit-weight table (the same one the Rebar Weight Calculator uses), not re-derived from diameter and steel density — that's what mill certs actually report.
Single mat vs. two layers
A single-mat slab (bottom bars only) resists positive moment. A slab that also needs to resist negative moment — commonly a slab on a supported structure rather than on grade — gets a matching mat near the top face too, which simply doubles the bar count, length, and weight. The "Mat Layers" option handles that without re-entering anything.
Limits
This assumes a single rectangular grid at a uniform spacing in both directions — it doesn't handle non-rectangular slabs, varying spacing (denser near a column, for example), or bars added around openings and edges beyond the standard cover. Waste/lap allowance is a flat percentage, not a per-lap-length calculation. Always check the result against your structural drawings and your project's actual specified cover and lap requirements.
Privacy
Everything runs in your browser — no dimensions or project data are sent anywhere.
Related tools
For a single bar's weight from its cut length and quantity, use the Rebar Weight Calculator. For the concrete itself, see the Concrete Volume & Bags Calculator.