# Rebar Anchorage Length Calculator

Basic and seismic anchorage length from bar grade, diameter and concrete class.

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

## Quick result

With the default inputs (Bar grade HRB400, Bar diameter 20 mm, Concrete class C30, Seismic detailing Non-seismic, Coating Uncoated): **Anchorage length la 705 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Bar grade | select | HRB400 | — |  |
| Bar diameter | number | 20 | mm |  |
| Concrete class | select | C30 | — |  |
| Seismic detailing | select | Non-seismic | — |  |
| Coating | select | Uncoated | — |  |

## Formula

- **Basic anchorage**: la = α × (fy ÷ ft) × d
- **Shape factor α**: 0.16 plain bars, 0.14 deformed bars
- **Corrections**: ζa = 1.10 (d>25) × 1.25 (epoxy coated)
- **Seismic**: laE = ζaE × la (1.15 for class I-II, 1.05 for III-IV)
- **Minimum**: la not less than 200 mm

## Worked example

HRB400, 20 mm, C30, non-seismic: fy = 360 and ft = 1.43 give la = 0.14 × 251.7 × 20 ≈ 705 mm, about 35 bar diameters.

## Questions

### How does anchorage length differ from lap length?

Anchorage length is the minimum embedment for one bar to develop its force through bond with the concrete. Lap length transfers force between two bars; each carries only part of it and a joint factor of 1.2-1.6 applies, so a lap is always longer than the anchorage.

### Why the 1.1 factor above 25 mm?

Large bars have a lower bond-area-to-section ratio and are harder to encase properly during placing, so bond is relatively poorer. The code compensates with 1.1. Congested heavy bars also tend to leave voids underneath, so vibration and aggregate size matter as much as extra length.

### Why longer anchorage in seismic design?

Under earthquake action, bars in joints may yield and even overstrength, exceeding design forces, while reversed cyclic loading degrades bond. Seismic detailing multiplies anchorage by 1.15 for class I-II and 1.05 for III-IV, with confining hoops. Frame joints are where this fails most often.

## 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 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.
- [Rebar Mesh Weight Calculator](https://jianzhujisuan.com/en/rebar-mesh): Weight per square metre and total mesh weight from dimensions and spacings.

---

Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.