Spring Constant Converter
Structural
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Formula
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What this converter does
This converter changes a spring rate from lb/in to N/mm and back, and also to N/m, kN/m and kip/in. Type a value, choose your units, and read the result instantly.
One lb/in equals 0.1751 N/mm. Swap the arrow to go from N/mm back to lb/in — the round trip agrees exactly.
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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 |
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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
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Worked example
Convert a 143 lb/in spring rate to N/mm.
Step 1 · Into N/m
143 lb/in × 175.13 = 25,043 N/mStep 2 · Into N/mm
25,043 ÷ 1,000 = 25.0 N/mmSo a 143 lb/in spring is about 25 N/mm.
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The units in this example
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
Newton per millimetresymbol: N/mm
A common metric stiffness unit for springs and machine elements. Equals 1,000 N/m or 1 kN/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
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FAQ
QHow do I convert lb/in to N/mm?
Multiply lb/in by 0.1751. One lb/in equals 0.1751 N/mm of stiffness.
QWhat is kip/in in N/mm?
One kip/in equals 175.13 N/mm, since a kip is 1,000 lb.
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