What it measures
Smash factor is a simple ratio: ball speed divided by club speed. A swing at 100 mph that produces 150 mph of ball speed has a smash factor of 1.50. It has no units and no direction, so it is the same for right and left handed golfers.
It is often called a contact quality number, but it really measures energy transfer, which depends on loft and delivery as well as where the ball hits the face.
Typical numbers
PGA Tour and LPGA Tour players average about 1.49 with driver, and a well struck driver tops out around 1.50 because equipment rules limit how springy the face can be. Tour 6 iron averages sit near 1.39 to 1.41, 7 irons around 1.33 to 1.34, and a good pitching wedge sits near 1.25.
Everyday golfers often see 1.40 to 1.46 with driver and around 1.25 to 1.35 with a 7 iron, with more variation shot to shot. Because iron lofts differ between sets, a high iron smash factor can partly reflect a strong lofted club rather than better contact.
What changes it
Center contact raises it, and hits toward the heel, toe, top or bottom lower it. Spin loft has a big effect: less spin loft, meaning less loft relative to the direction the club travels, produces a more direct hit and a higher smash factor, while more spin loft produces a more glancing hit and a lower one. A large face to path difference also turns some energy into sideways spin and lowers it.
Equipment fit, ball type and loft all play a part, which is why each club has its own expected range.
What it changes
Smash factor directly sets ball speed for a given club speed, so a small change is worth real distance. Going from 1.42 to 1.48 at 100 mph of driver speed adds about 6 mph of ball speed, which can be worth more than 10 yards. Low smash factor often comes with extra spin, because the glancing, high spin loft strikes that cost ball speed also add backspin.
How to use it
Judge smash factor over groups of shots and against the same club. If it is low, check impact location first, then spin loft (dynamic loft and attack angle) and face to path.
A useful bay drill is the half speed smash test: hit five shots at about 70 percent effort and note your smash factor, then build up effort in steps while trying to keep the ratio the same. The point where smash factor drops is where your swing starts to lose control of contact, and it is a good speed to practice at until contact holds up.