For 45 ACP 1911s, 16 lb is basic stock. 18 lb is what many people replace their 16 lb springs with. If you shoot light target loads, consider 14 lb, if you shoot powder puff, consider 12 lb. If you have a Commander slide or shorter, start at 20 lb, I ended up with 21 lb in my shorty. Recoil springs shorten over time. Replace yours when it has lost 1/2 inch of it's original length. That's right, measure them before you install 'em. Make certain you replace your firing pin spring at the same time. You do not have to buy springs from Ruger, both Wolff and ISMI make superb springs, and most suppliers buy their springs from one of these two companies. Untold millions of words have been written over 1911 springs.