Search for how to measure for a baseball bat and you will find the same three tests on every page. Chest to fingertips. Arm reach. Bat against the hip.
Run all three on the same kid, and you will often get three different answers.
Nobody explains what to do about that, so here is the honest version. The tests are a starting point. They are not the answer.
The three tests everyone publishes
The chest-to-fingertips test. Extend your arm straight out to the side, parallel to the ground. Measure from the center of your chest to the end of your fingertips. That number in inches is your starting bat length.
The arm-reach test. Put the knob of the bat in the center of your chest and point the barrel straight out to the side. If your fingers can reach the end of the barrel, the length works.
The hip-and-ground test. Stand the bat upright next to your leg with the knob up. The barrel should reach around the hip. Some versions say you should be able to grip the handle without leaning over.
They all measure the same idea, which is proportion. How long is this bat relative to how long this kid is.
Why they disagree
Arm length and torso length are not the same measurement, and kids are not built to a formula.
A player with long arms and a short torso passes the arm-reach test at a longer bat than the hip test allows. A player built the other way gets the opposite result. Neither kid is unusual. They just are not average, and the tests assume average.
The hip version is also inconsistent between sources. Some say the barrel should reach the hip bone and stop. Others say you should be able to grip the handle standing straight. Those are different bats.
So when they disagree, you have not done anything wrong. You have just found the edge of what a proportion test can tell you.
The fourth test, and the one that matters in a game
Lay the bat across home plate.
Stand where your player stands in the box, put the bat out over the plate, and see whether they can cover it. Can they reach the outside corner without lunging?
That is the only test that measures the thing the bat has to do in a game. Everything else measures how long the kid is.
A bat that passes all three proportion tests and cannot cover the outside corner is too short. And a bat that covers the plate but the kid cannot get around in time is too long, which brings us to the part the tests do not touch at all.
What the tests cannot tell you
Control.
Length and drop weight and barrel size are useful numbers. They are not the whole picture, and they get treated like the whole picture on almost every page that covers this.
A bat can measure correctly and still be the wrong bat, because measuring says nothing about whether the player can move it where they want to.
Five to ten swings
Here is what actually settles it.
Have them take five to ten real swings. Not one. Not a wave in a store aisle.
Grip comes first. Different manufacturers use different grip tape and different handle thicknesses, and it matters more than people expect. If the handle feels wrong in their hands, the rest does not get a chance. One thing worth knowing here, and most kids get it wrong: you are not supposed to squeeze the handle tight.
Weight shows up within two swings. If it feels heavy on swing two, it is heavy. That does not improve. Put it down.
Length shows up in whether they can get around. If the bat is long enough that they are still dragging it through when the ball would be past them, the length is wrong regardless of what the tape measure said.
The tradeoff, stated plainly: a lighter bat that does not travel as far beats a heavier bat they cannot control. Every time, at every age.
If your player is young and has no idea yet
An older player knows what kind of hitter they are. A nine-year-old does not, and neither do you yet.
For that player, lighter is better. Full stop.
The thing to watch for is a kid swinging the bat like a tennis racket. Arms only, no hips, the barrel trailing behind. That is what too heavy looks like on a young player, and you do not need to know anything about baseball to spot it.
Comfort is the whole test at that age. If they look comfortable taking a cut, the bat is close enough.
What a good swing looks like
You are watching four things.
The hips are moving. Hips drive the swing, and a kid who is arms-only is either fighting the bat or has not learned the move yet.
The bat goes through the zone rather than around it.
The head stays down and the eyes stay on the ball.
And there is not a lot of extra movement before the pitch.
That last one is worth expanding. A lot of kids shake the bat, add a leg kick, or copy a stance they saw on TV. Major leaguers do those things because they have the timing to make them work. A young player usually does not, and by the time the pitch arrives they cannot get the barrel through.
Less movement is better while they are learning. You cannot push a stance onto someone, and a player has to figure out what works for them, but a quiet setup gives them a better chance of being on time.
The store problem
Swinging a bat in a sporting goods store is not the same as swinging it in a game.
In the store they are fresh, standing still, and nobody is throwing anything. In the sixth inning, tired, against a kid throwing hard, that same bat is a different bat.
Whatever you can do to test it under something closer to real conditions is worth doing. Cage time. A tee in the driveway. Borrowing one from a teammate for a practice. The fundamentals work off a tee at any age, and a tee will tell you more about whether a bat fits than a store aisle will.
The question to ask your kid
After the swings, ask them why they like it.
If they can tell you, in whatever words a nine-year-old uses, that is real information. It feels light. I can get it around. It feels good in my hands.
If they picked the first bat they touched and cannot say why, that is not a choice. That is a color they liked. Go back and take more swings.
Length limits your league sets
Sizing is only half the question. The other half is what your league allows.
Little League caps bat length at 33 inches for Major and below, and 34 inches for Intermediate, Junior and Senior divisions. Those limits are published on Little League’s bat information page, and they apply no matter what a sizing test tells you.
The certification matters too. A bat that fits perfectly and carries the wrong stamp is unusable. USA Baseball maintains the approved bat list for USABat divisions, and USSSA publishes its own rules for travel play.
Two things worth doing before money changes hands. Confirm which stamp your league requires, and if you are buying used or on clearance, check the model against the decertified lists, because a bat can come off the approved list after it goes on sale.
Common questions
Which of the three tests is most accurate? None of them is definitive. Use them to narrow to a range of two or three lengths, then let the swings decide.
What if the tests give me different lengths? Start with the shorter one. A slightly short bat a player controls will outperform a slightly long bat they do not.
Do I need a tape measure? No. The arm-reach and hip tests need no equipment, and the swings matter more than either.
How do I know if a bat is too heavy? Two swings. If it feels heavy, it is heavy. You can also use the arm-extension check: hold the bat straight out to the side for 20 to 30 seconds. If the arm drops fast, it is too much bat.
What size should my player be swinging by age? The calculator here takes height and weight and returns a length and drop weight, and the size chart breaks it down by age. Both are starting points for the same reason the tests are.
What is drop weight? The difference between length in inches and weight in ounces. Full explanation here.
Where do the league limits come from? Little League bat information, USABat approved bats, and USSSA baseball rules. Rules change. Check the current version before you spend money.
Not sure which certification your league requires? Check your bat here.
Want a starting length based on height and weight? Run the calculator.