Buying your first kilt is something quite unusual. Most likely you shall buy it online, and often it shall be custom made. Most likely this is the first time in your life where you have to take or have taken measures with the purpose of having something sewn to really fit you. It is not difficult, however, and with a kilt there is some give and take; in contrary to trousers.
The measures
When ordering a kilt your are asked for up to five measures. They are waist size, There are maximum five measures , they are:
Waist size - Fell length - Hip size - Kilt length - Your height.
Sometimes it is only waist and length, and do you order a ready-made kilt, you only have to inform about your waist size, the length being with 95% probability 24 inches or 61 cm. But then of course, you still have to find out, whether this length shall fit you, but let's take the measurements step by step.
They use yards, feet, and inches
To a continantal European it might seem odd, but it is the way it is. And you already know it from jeans. Americans and Brittans don't use the metric system.
Fortunately, a recalculation is not difficult.
The basic measure is the inch.
1 inch (1 in or 1") = 2.54 cm
12 inches make a feet
1 feet (1 ft in or 1') = 30.5 cm
3 feet or 36 inches make a yard
1 yard = 91.5 cm
WAIST SIZE
A traditional kilt is supposed to sit about navel (your “natural” waist), i.e. higher than most trousers. A casual kilt can be worn lower, however.
Take your measure where you want your kilt to sit.
Don’t rely on your trouser size!
The clothing industry is pleasing their customers by lying about facts. My Tommy Hilfiger jeans waist size is 32” but my kilt waist is 36”!
A kilt being fastened by means of straps and buckles means that there is some flexibility, like plus/minus 1 inch. On off-the-peg kilts the flexibility might be even bigger, like for example 34”-38” = 10 cm.
FELL LENGTH
Like mentioned earlier the fell on a kilt is the part where the pleats are sewn down. In most cases you are not asked about this measurement. In fact, it is only when ordering a made-to-measure kilt in Pakistan. If you don't give an answer they'll make it 6" from the rise or 6.5" from the top of the kilt, no matter the length of it. It might be a bit to the short side, but probably perfect if you want your casual kilt to sit low. Shorter or longer is up to you, and when you have the option, why not use it? The fell should end where you are at the broadest, i.e. where you'll take the next measurement. Better too short than too long.
HIP OR SEAT SIZE
It is where you are at the broadest. Take a lose measurement.
When buying a readymade kilt you are never asked. Then the seat size is based upon the waist size.
KILT LENGTH
The kilt should at the longest end at the middle of your knee cap. By no means should it cover your knees – even if that not uncommon. At the very shortest it should be one inch above the knee. That means a give and take of about five cm or two inches.
Should you get across the renowned book,
So you*re going to wear the kilt by
J. Charles Thompson, you’ll see that the author is advocating for short kilts; and that for special purposes they might even be quite short.

J. Charles Thompson: So you're going to wear the kilt, 3rd revised edition 1989, Lang Syne Publishers LTD, Glasgow.
At the very longest, the kilt should reach only to the top of the knee cap… Don’t get the kilt too short, either. One of the books tells of a Highland gentleman who has his heavy kilt that he wears for shooting and hillwalking three inches above his knees. That way it does not touch the calf of his leg at all. That may be all very well if you are going to have a kilt, especially for outdoor sports, but for a general-purpose kilt an inch above the top of the knee cap is probably as short as you will want to go, even if you like a shortish kilt. J. Charles Thompson: So you're going to wear the kilt, 3rd revised edition 1989, page 54.
And further he says:
Don’t get your kilt too long! The top of the knee cap is the absolute long limit, and up to an inch shorter is acceptable – better in my personal opinion.
J. Charles Thompson: So you're going to wear the kilt, 3rd revised edition 1989, page 99.
A few examples:
A kilt is a most flexible garment. Below are six different kilts, ranging from 24" down to 20", meaning a difference of no less than 10 centimeters, and five of them can be worn according to rules.

Too long. Kilt covering the knees. If worn higher OK. 24" 8 Yard PV ready-made kilt. MacKenzie tartan.

No longer than this. Mid knee cap. Kilt worn at natural waist. 24" 5 yard wool kilt made-to-measure. Douglas Green Modern tartan.

Top of knee cap. The kilt length I prefer. 22.5" 5 yard wool kilt made-to-measure. Campbell Ancient tartan.

About one inch above the knee. Better no shorter than this. 22.5" sitting a little higher than on the kilt on the picture above. 5 yard wool kilt made-to-measure. Holyrood tartan.

About two inches above the knee. Too short, if, like here, worn at natural waist. If lowered as munch as it can, it shall still be acceptable. 21" 3 yard PV off-the-peg kilt. Buchanan Antique tartan.

The same kilt, worn lower and now just within limits.

A cheap 5 yard 20" made-to-measure PV kilt. Ordered that short from an American vendor. Stewart Black tartan. Here worn high but the kilt just a bit shorter than the length, which J. Charles Thompson finds still acceptable for a kilt, "especially for outdoor sports". I think most men should consider it too short.

The same kilt, worn as low as it could. The length now is fine, and the kilt looks OK - as long as the top is covered by a jacket or a sweater, but otherwise it is too short (to be authentic) and is looking more like a low-waist miniskirt.

Don't get your kilt TOO short. Especially in countries where school uniforms are common, a short kilt might be associated with schoolgirl's pleated skirts (Tokyo, Japan and Malaga, Spain). Also in Scottish environments where kilts are well known, better be within accepted tolerances.
Taking the measure
I have never been able to use the "kneeling" method" often referred to.
My advice:
Use a camera or your smartphone, a towel and a belt. The phone or camera should be placed in a vertical position, meaning parallel to the wall and in knee height.
With the belt at navel – or where you should like to fasten your kilt – adjust the towel to the 24” standard length.
With you standing upright (like with your back against the wall) take the picture with the self-timer. If not satisfied adjust the length of the towel until it looks right on the photo and take the measure.
You might also use the smartphone on a selfie stick. Just NEVER lean forward when taking the picture!
Another way is to have your wife or girlfriend assist you.
Be aware that a custom made kilt cannot be returned, just because it does not fit you! A ready-made 24" long kilt can, even if it might have become more complicated to return a kilt from an EU-country to Scotland or wherever it has come from.
Placed at navel a 24” kilt goes to the middle of my knee cap. In my case this length is the absolute maximum. A 21” kilt resting on my hips comes to about one inch above my knee cap, thereby still just acceptable. Fastened at navel it should look more like a mini kilt, however. The perfect kilt length is for me 22.5"-23”. This just to indicate that there is some flexibility – and come in mind we are talking casual kilt wearing, not national dress. I’m 176 cm tall.
For me, better too short than too long.
Rule-of-thumb
If you are of average build, you can expect a kilt being around a third of your height to fit.
Being 176 cm = 69", a third is 23". 23" is for me the perfect length = mid/upper knee cap. Preferring my kilts to be on top of my knees, I order them 22.5".

My preferred kilt length. 5-yard kilt, pure new wool, 22.5" long. Gunn Ancient tartan, Helsingborg, Sweden.
YOUR HEIGHT
Not all vendors will ask. You are supposed to give your height in feet and inches. It is just to give an impression, whether the length you have ordered seems correct. Varying by 2.54 cm it is not a very precise indicator, but if you order your kilt 20” (51 cm) short and your height is 6’9” (206 cm) you
might be asked whether you really want it to be a micro mini kilt?
1 foot = 30.5 cm.
My 176 cm equal 5' = 152.5 cm plus 23.5 cm = 9", meaning I am 5'9" high.