9 gauge (3.6mm) zigzag (no-sag) springs. Zigzag springs are commonly used on seat bases. They should be fitted under tension so they’re almost flat, with just a slight arc, spacing between 2-3 inches apart, the smaller the gap between springs, the firmer the seat will feel. Once fitted bracing wire or cord can be attached so the springs function as a unit & spread the loading. The 9 gauge is the most common, for a firmer base we do sell the 8 gauge (4.00mm) To get an accurate measurement of the length of the spring, press the spring flat and measure the top of the spring.