I will make is simple

A hand is 4 inches.
There are therefor 3 hands to a foot.

So a 15 hand horse is 5 feet tall at the withers.
An 18 hand horse is 6 feet tall at the withers.

If you are 5 feet tall, and your horse is shorter than you, then your horse is 15 hands or less (allowing for the fact you are likely wearing some kind of shoes as you stand beside your horse).

if you are 5 foot 3 inches, and wearing runners, and can easily see over your horse's withers, then your horse is not 15.2 hands unless you have eyes on the very top of your head.

Measuring sticks are not expensive, and good ones have levels to make it simple.

If you do not have a measuring stick, then stand your horse square on level ground, hold a level on its withers (hold the level level!) and measure from the ground to the level.

If you are unable to measure your horse, then put in the ad: "I think my horse is 15 hands, but we have not measured her."

Do not blindly accept what the person who sold you the horse before told you.

Even vets make mistakes, so don't accept their word unless you saw them measure your horse and feel comfortable with the technique used. I remember when YJ was first set to come, his owner told me he was likely 15.3 hands, based on him being measure as 15.2 hands by the vet the year before (and she showed me the vet report to prove it). He was 15.1 hands when he came. Horses do not shrink between their 3 year old and 4 year old year.

Any questions?

Karen