Mattress Spring Wire Diameter (mm) vs Gauge: Conversion Table

6 October 2026

Last updated: 6 October 2026

Short answer: mattress spring wire is specified either as a diameter in millimetres or as a gauge number. The gauge used for spring steel wire is the Washburn & Moen (US Steel Wire Gauge) scale, not AWG: a lower gauge means thicker wire. For example 13 gauge = 2.324 mm and 15 gauge = 1.829 mm. KAR-AS specifies its spring units in millimetres.

Conversion table (Washburn & Moen steel wire gauge)

Gauge	Diameter (mm)	Diameter (inch)	Where KAR-AS ranges fall
10	3.429	0.1350	–
11	3.061	0.1205	–
12	2.680	0.1055	–
13	2.324	0.0915	Bonnell units: 2.20–2.38 mm
14	2.032	0.0800	Pocket units, upper end: 2.10 mm
15	1.829	0.0720	Pocket units
16	1.587	0.0625	Pocket units
17	1.372	0.0540	Pocket units
18	1.206	0.0475	Pocket units, lower end: 1.20 mm
19	1.041	0.0410	–
20	0.884	0.0348	–

How to read wire diameter in a specification

Thicker wire (lower gauge) gives a firmer coil at the same height and coil geometry; thinner wire gives a softer coil.
Firmness also depends on coil height, number of turns, coil diameter and how many coils are in the unit (the count), so wire diameter alone does not define the feel.
Bonnell units use thicker wire than pocket units because fewer, larger coils carry the load together.
Values between two gauges are normal: specifying in millimetres (for example 1.40 mm or 2.20 mm) avoids rounding.

KAR-AS wire diameters at a glance

Unit type	Wire diameter	≈ Gauge	Height
Bonnell spring unit (KAR-AS)	2.20–2.38 mm	≈ 13	100–180 mm
Pocket spring unit (BESTTEL)	1.20–2.10 mm	≈ 18–14	6–28 cm

Model-by-model values are in the technical tables on the Bonnell and pocket spring pages. Related: pocket vs Bonnell comparison, glossary. Conversion: 1 inch = 25.4 mm; diameters rounded to 0.001 mm.

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