How to Read a Chronograph: Average, ES and SD Explained

A chronograph gives you three numbers for a string of shots: average velocity (where the load shoots), standard deviation, or SD (how consistent it is — the one that matters at distance), and extreme spread, or ES (your fastest shot minus your slowest, useful mainly as a flier check). Read them in that order of importance and you won’t be led astray.

The screen throws a lot at you, but almost everything reduces to those three. Here’s what each one is actually telling you, and which ones to trust when you’re choosing a load.

Average velocity — where the load sits

The average (mean) is just all your shot velocities added up and divided by the shot count. It tells you how fast this load runs, which you need for two things: staying within a safe, sane speed for your cartridge, and feeding a ballistic solver so your drops at distance are right. Average alone says nothing about consistency — two loads can share an average and behave completely differently shot to shot.

Standard deviation — the one that matters

Standard deviation describes how tightly your shots cluster around that average. A low SD means the load throws each bullet at nearly the same speed; a high SD means the velocity wanders. It matters because velocity spread becomes vertical spread downrange — a slow shot drops more, a fast one less — and that stretches your group taller the farther you shoot. At 100 yards a sloppy SD hides; at 600 and beyond, SD basically is your vertical. That’s the number to chase. What counts as a good SD →

Extreme spread — the flier check

Extreme spread is the simplest number: fastest shot minus slowest. It’s easy to picture, but it’s set entirely by your two most extreme shots, so one bad round doubles it while the load itself never changed. Don’t chase ES — read it as a flier detector. If your ES is wide but your SD is tight, you probably had one odd shot, not a bad load.

The number the screen doesn’t show: sample size

Here’s the trap. Those three numbers are only as honest as the shot count behind them. An SD off three or five shots is mostly luck and settles down only once you’ve fed it real rounds. So a “single-digit SD” on a three-shot string tells you almost nothing. Before you trust a chronograph number for a real decision, give it enough shots to mean something.

Turning the numbers into a load decision

Reading the screen is step one; the real work is comparing loads on all of it — average, SD, and the group they printed on paper — and that’s more than a chronograph does. Loadscope reads your chronograph data (Garmin Xero, LabRadar, MagnetoSpeed and more), computes the average, SD and ES for every charge, lines them up against your target groups, and scores them to name your best load. See how the scoring works →