Spring Constant Converter
Structural
0
Formula
01
What this converter does
This converter changes a spring rate or stiffness from N/mm to lb/in and back. It also covers N/m, kN/m and kip/in. Type a value, choose your units, and the result appears as you type.
Stiffness is force per unit deflection. One N/mm equals 5.710 lb/in, so results are exact factor conversions, not estimates.
02
The units it covers
Stiffness is force per unit deflection, so each unit converts through N/m by a fixed factor.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Newton per metre | N/m | 1 | SI base unit |
| Newton per millimetre | N/mm | 1,000 | Machine & spring specs |
| Pound per inch | lb/in | 175.13 | US spring ratings |
| Kilonewton per metre | kN/m | 1,000 | Structural stiffness |
| Kip per inch | kip/in | 175,127 | US structural stiffness |
03
The formula
Each stiffness unit has a fixed value in N/m, so the conversion is one multiplication:
Conversion
result = value × factor_from ÷ factor_toWhere:
- value = the stiffness you typed
- factor_from = its value in N/m
- factor_to = the target unit’s value in N/m
04
Worked example
Convert a 25 N/mm spring rate to lb/in.
Step 1 · Into N/m
25 N/mm × 1,000 = 25,000 N/mStep 2 · Divide by lb/in factor
25,000 ÷ 175.13 = 142.8 lb/inSo a 25 N/mm spring is about 143 lb/in.
05
The units in this example
Newton per millimetresymbol: N/mm
A common metric stiffness unit for springs and machine elements. Equals 1,000 N/m.
Common stiffness conversions
- 1 N/mm = 1,000 N/m
- 1 N/mm ≈ 5.710 lb/in
- 1 N/mm = 1 kN/m
- 10 N/mm ≈ 57.1 lb/in
Pound per inchsymbol: lb/in
The US customary spring rate — pounds of force per inch of deflection.
Common stiffness conversions
- 1 lb/in ≈ 0.1751 N/mm
- 1 lb/in = 175.13 N/m
- 100 lb/in ≈ 17.51 N/mm
- 1 kip/in = 1,000 lb/in
06
FAQ
QHow do I convert N/mm to lb/in?
Multiply N/mm by 5.710. One N/mm equals 5.710 lb/in of stiffness.
QIs N/mm the same as kN/m?
Yes. One N/mm equals exactly one kN/m, since both are 1,000 N/m.
08