Measuring for blinds sounds simple until you are standing at the window with a tape measure, wondering which number actually matters. Measure wrong and the blind either will not fit in the opening or hangs with light gaps down both sides, and custom blinds cannot be returned for a redo.
I measure windows for a living all over the Des Moines metro, so I have made every one of these calls thousands of times. Here is the same process I use, step by step, whether you are ordering blinds yourself or just want to understand exactly what I am doing during an in-home estimate.
Inside mount vs outside mount
Before you measure anything, decide where the blind will live. An inside mount sits inside the window opening, tucked up close to the glass. It gives you that clean, built-in look, shows off your trim, and keeps the blind out of the way of curtains or furniture. It is what most people picture when they think of blinds.
An outside mount hangs on the wall or trim above the window, so the blind covers the whole opening from outside it. You give up the recessed look, but you gain coverage. Outside mounts hide shallow windows, out-of-square openings, and cranks or handles that would block an inside mount, and they block more light around the edges too.
The choice matters because the two mounts are measured completely differently, and mixing up the rules is the most common mistake I see. Inside mounts need exact opening sizes so the factory can make its deductions. Outside mounts need you to decide the finished size of the blind yourself.
Measuring for inside mount blinds
For an inside mount, measure the exact width of the opening in three places: top, middle, and bottom. Windows are rarely perfectly square, especially once a house has settled, so use the narrowest of the three numbers. The factory will take a small deduction from that width so the blind operates without scraping the frame.
Measure the height the same way, at the left side, center, and right side, and use the longest of the three so the blind reaches the sill everywhere. Record everything to the nearest eighth of an inch, and never round up on width. An eighth of an inch too wide means the blind does not fit at all.
Check your depth too, since every blind needs a minimum depth for its brackets and a little more to sit fully recessed. Then give the manufacturer the exact opening measurements and let them make the deductions. Deducting on your own is the classic do-it-yourself error, because the factory deducts again and you end up with daylight down both sides.
Measuring for outside mount blinds
Outside mounts flip the logic. You are not measuring an opening anymore, you are deciding how much wall or trim you want covered, and the number you give the manufacturer becomes the exact finished size of the blind.
Measure the width of the area you want covered, and plan on overlapping the window opening by a couple of inches on each side for privacy and light control. For height, start above the opening where the headrail will mount and measure down to the sill, or past the frame if there is no sill and you want the blind to hang below it.
Mark where the brackets will land and make sure you are drilling into trim or solid backing rather than bare drywall. Then step back and look at the whole wall before you commit. On a wall with several windows, keeping every headrail at the same height makes the room feel finished, even when the windows themselves do not quite match each other.
Why older Des Moines homes are tricky
A big share of my work is in established neighborhoods, in houses that have stood through ninety or more Iowa winters. I love working in those homes, but their windows keep you honest. Openings that measure a half inch wider at the bottom than the top are routine, and sills can slope enough that a perfectly built blind looks crooked if you only measured in one spot.
Older homes also bring shallow window pockets, layers of paint that quietly narrow the opening, original casings nobody wants to drill through, and storm windows or locks that steal usable depth. Every one of those details changes the right mounting call, and sometimes the honest answer is an outside mount even when you had your heart set on the recessed look.
None of this means older windows cannot look fantastic with the right treatment. It just means the measuring stage deserves extra patience, more measurements per window, and a willingness to treat every single window as its own little project.
Tools that make it easier
You do not need much, but the right tools remove most of the error. Use a steel tape measure, never a cloth sewing tape, which stretches and sags across a wide opening. Write every number down the moment you take it, labeled by window and marked as width or height. Trust me, ten minutes later every window in the house blurs together.
Record measurements in width by height order every time, because reversed numbers are one of the most common ordering disasters in this business. A small step stool helps you get level with the top of tall windows instead of angling the tape upward, which distorts the reading. And measure everything twice. The second pass costs two minutes and catches the mistake that would have cost you a blind.
If a window has anything odd going on, a crank handle, an alarm sensor, tile running inside the opening, write that down too. Those details decide bracket placement, and it is far better to discover them before order day than on install day.
Or skip the tape measure entirely
Here is the part where I make it easy on you. Measuring is the one step in this whole process where a small mistake becomes a permanent problem, because custom blinds are cut to your numbers and cannot be resized afterward. It is also the step I will happily take off your plate for free.
I measure every window myself at every estimate, and when I do the measuring, the fit is my responsibility instead of yours. If something is off, I make it right. You can see exactly how the process works from first visit to final install, and browse the custom blinds we build to fit everything from a brand new build to a foursquare near downtown.
Estimates are free, there is never any pressure, and we are open seven days a week, evenings and weekends included. Send us a note or call (515) 850-9700, and I will be the one who shows up at your door with the tape measure.
Have a question I did not cover? Call (515) 850-9700 or request your free in-home estimate and I will give you a straight answer for your exact windows.