How to Measure a Window for Replacement: A Minnesota Contractor's Step-by-Step

I've been measuring windows in Twin Cities homes for decades, and I'll tell you right up front — most homeowners don't need to measure their own windows. That's my job. But I get the question all the time: "Joe, two contractors gave me wildly different quotes. Were they even measuring the same window?"

Fair question. Here's how I'd answer it.

This guide walks through the method I use when I'm standing in your dining room with a tape. The point isn't to turn you into an installer. It's so you can sanity-check a quote, spot a contractor measuring sloppy, or size a few inserts yourself without ending up half an inch off.

Half an inch matters. In a Minneapolis 1950s rambler with a settled foundation, it can be the difference between a window that seals and a window that whistles all winter.

The Short Answer (If You're Skimming)

Measure the width at three places — top, middle, bottom — and write down the smallest number. Measure the height at three places — left, center, right — and write down the smallest number. Then measure depth front-to-back inside the frame. Always to the nearest 1/8 inch. Always width first, then height. Always the smallest of the three.

That's the whole method. The rest of this post is the details that trip people up — sloped sills, egress code, when "insert" is wrong, and the Twin Cities-specific stuff I've watched homeowners get burned on.

Tools You Actually Need

Three things:

• A 25-foot steel tape measure with a sturdy lock. Not the little 12-footer in your junk drawer.
• A 4-foot level. Torpedo level is fine if that's what you've got.
• A pen and paper, or notes on your phone with a sketch you can label.

Skip the laser measure. They bounce off the wrong surface in residential frames and read long.

Step 1: Width — Three Measurements, Take the Smallest

Open the window. Push the lower sash up so the tape reaches the side jambs.

Measure horizontally jamb to jamb — not trim to trim. The jamb is the inside vertical surface of the frame. The trim is the decorative casing nailed to the wall around it. Measure the trim and your number's too big — the window won't fit.

Three readings:

1. Top — just below the head jamb.
2. Middle — where the meeting rail sits when the window is closed.
3. Bottom — just above the sill.

Write all three. Circle the smallest. That's your width.

Why smallest? If the opening is even a hair narrower at one point than your window, it won't fit. Period. You're not averaging.

In Twin Cities homes built before 1990 — which is most of Edina, St. Louis Park, Hopkins, and older Minneapolis — the three width measurements are almost always different. A 1/4-inch spread is common. 1/2 inch isn't unusual. That's 60+ years of frost-cycle settling.

Step 2: Height — Three Measurements, Take the Smallest

Same idea, rotated 90 degrees. Measure vertically from the head jamb (inside top of the frame) to the sill (inside bottom).

1. Left — up the left jamb.
2. Center — straight up the middle.
3. Right — up the right jamb.

Write all three. Circle the smallest. That's your height.

One thing to flag: the tape has to sit flat against the jamb. If it curves outward because the sill is sloped, you'll read long. Hook the tape on the head jamb, let it hang straight, and read where it meets the sill — not where the tape body rests.

Step 3: The Sloped Sill Quirk

Window sills aren't flat. They slope down and away from the house — about 10 to 15 degrees — so water runs off instead of pooling against the wall.

That means the sill is lower at the front edge (room side) than at the back (exterior side). If you measure to the front of the sill, you'll read 1/4 to 3/8 inch taller than the actual frame height the window has to fit into.

The fix: measure to the highest point of the sill — the back edge against the exterior siding. That's where the new window's bottom will actually sit. If your existing sash blocks the tape, run a level across the sill, find the high point, mark it with a pencil, and measure to your mark.

This one detail is why I tell homeowners not to fully trust their own height numbers. A good contractor catches this. A volume installer measuring 20 homes that day might not.

Step 4: Depth — Is the Pocket Even Deep Enough?

Most insert replacement windows need at least 3 inches of usable depth inside the existing frame.

Measure from the inside edge of the room-side stop to the outside edge of the exterior stop. 3 inches or more, insert is usually viable. Less than 3 inches — common on 1940s and earlier homes around Linden Hills, Excelsior, and the older Lake Minnetonka cottages — and you're likely looking at full-frame.

Also note any interference: interior trim, sash locks, storm window tracks, exterior shutters. If your contractor doesn't ask about depth or interference, that's a flag.

Egress Code — The Other Measurement That Matters

Minnesota code requires any window in a sleeping room (including basement bedrooms) to meet egress requirements — meaning a person can get out of it in a fire. Not optional. Replace a bedroom window with one smaller than code allows and you've created a violation that surfaces at the next sale.

The minimums to know:

• Minimum net clear opening: 5.7 sq ft (at-grade basements: 5.0 sq ft).
• Minimum opening width: 20 inches.
• Minimum opening height: 24 inches.
• Maximum sill height from finished floor: 44 inches.

"Net clear opening" is the actual hole the window leaves when fully open — not the frame, not the rough opening. A double-hung only opens to about half its total height, which is why a 36×60 double-hung often doesn't meet egress even though the rough opening looks plenty big.

Measuring a bedroom window, write down your numbers and ask the contractor: "Does this meet egress for a bedroom?" Vague answer = different contractor.

Insert (Pocket) vs. Full-Frame — What You're Actually Measuring

Same numbers. Different frame you're measuring into.

Insert replacement (pocket replacement): the existing exterior frame stays, new window slides in. You're measuring the inside of the existing frame. The new window will be 1/4 to 1/2 inch smaller than your numbers so it shims and seals. Insert is the right call when the frame is dry, solid, and square. About 70% of the windows I replace in Twin Cities homes are insert jobs.

Full-frame replacement: existing frame, sill, and trim all come out. New window goes into the rough opening itself. The right call when there's rot, severe air leakage from the frame (not just the sash), or when you want to change the window size. Typically runs $600 to $1,000 more per window.

If a contractor quotes full-frame on every window without explanation, ask why. Sometimes it's right. Sometimes it's an upsell. Pull a piece of interior trim near the sill on one window and look — dry wood means insert is fine; dark, soft, or crumbly means full-frame on at least that opening.

When to Trust Your Own Numbers — and When Not To

Trust your numbers to sanity-check a quote. If a contractor's numbers come back within 1/4 inch of yours, they measured carefully. Off by 1/2 inch or more on multiple openings, ask them to re-measure. A reputable installer will. A volume shop will say "we'll adjust on install day" — usually code for "we hadn't measured yet."

Don't trust your numbers to actually order windows. Manufacturer-direct ordering puts the fit risk on you. A window 1/4 inch too tall is a $400-$800 mistake before labor. Kolbe, Pella, and ProVia all require the installing contractor to take final measurements. If a "contractor" says they'll install windows you ordered yourself, that's a labor-only installer dodging warranty responsibility.

The Twin Cities Gotchas I See Every Year

Settled foundations on 1940s–1970s ramblers. Hopkins, St. Louis Park, Richfield, Bloomington. Openings aren't rectangular anymore. Top and bottom widths can be 1/2 to 3/4 inch different. Smallest measurement rules.

Lake home additions with non-standard openings. Minnetonka, Excelsior, Wayzata. One-off sizes no manufacturer stocks. You're custom-ordering through Kolbe (longer lead time, exact fit) or going full-frame and resizing the rough opening.

Wood-windowed '80s and '90s homes with rotted sills. Most common western-suburbs gotcha. Exterior looks fine. Probe with a screwdriver before you commit to insert pricing — if it sinks in, that opening needs full-frame, full stop.

Walkout basement egress windows. Eden Prairie, Chanhassen, Maple Grove finished basements. The well dimensions matter as much as the window — minimum 9 sq ft, minimum 36 inches projection. A contractor who doesn't ask about the well doesn't know the code.

A Quick Note on Recording

For every window, write down:

• Room and location ("Main floor, east-facing dining room, north window")
• Width — top / middle / bottom, smallest circled
• Height — left / center / right, smallest circled
• Depth — inches in the pocket
• Existing type — double-hung, casement, slider, picture, awning
• Notes — sloped sill confirmed, any rot, interference, is it a bedroom (egress), above a kitchen sink (operability)

One window per page, taped under the window. When the installer arrives, verification takes 30 seconds per opening instead of measuring from scratch.

When You'd Rather Not Measure 17 Windows Yourself

Whole-house replacements run 12 to 20+ windows. Three width readings, three height readings, depth, and notes for each. That's an evening of work, plus the risk that one number's wrong and you don't catch it until windows ship.

I'd rather come measure. It's free. I bring a real tape, a 4-foot level, and decades of knowing where Twin Cities houses lie about their dimensions. I write up a line-by-line quote — lump sum per window, scope detailed — and you compare against other quotes apples to apples. No pressure. No "today only" pricing.

Call 952-206-6339 or request your free estimate online.

Related reading:

• Window Replacement in Minneapolis — measurement visit through install day.
• Window Replacement Cost in Minnesota — installed pricing by type and brand.
• ProVia Windows, Pella Windows, Kolbe Windows — the three brands we install.
• Do You Need a Permit to Replace Windows in MN? — usually yes for full-frame, usually no for insert.
• Is It Cheaper to Replace All Windows at Once? — phased vs. whole-house math.

FAQ

What's the most common mistake homeowners make when measuring windows?

Measuring to the trim instead of the jamb. The trim sticks out past the actual opening by 1/2 to 1 inch on each side. Trim-to-trim makes your width 1 to 2 inches too big and the replacement window won't fit. Always measure jamb-to-jamb — tape against the inside vertical surface of the frame itself.

Should I measure in inches or in fractions?

Inches and fractions, to the nearest 1/8 inch. Don't round to the nearest whole inch. If you measure 35-3/8 inches, write 35-3/8, not 35-1/2. Manufacturers order in 1/8-inch increments, and your installer will subtract roughly 1/4 to 1/2 inch from your smallest measurement to get the actual window size that ships.

Do I list width or height first?

Width first, always. "36 by 48" means 36 inches wide and 48 inches tall. If a quote shows numbers without labels, the first one is width.

What if my three measurements are very different from each other?

Some spread is normal in older Twin Cities homes — 1/4 inch is common, 1/2 inch isn't unusual on 60+ year-old houses. More than 3/4 inch means significantly out of square, and you should plan on either full-frame (so the rough opening can be reshaped) or extra trim work to hide the gap on an insert. The smallest measurement still rules.

Can I measure for a basement egress window myself?

You can measure the window opening, but you also need to measure the well — minimum 9 sq ft, minimum 36 inches projection from the foundation, and a ladder if the well is deeper than 44 inches. Egress is a code issue, not just a window issue. If a contractor doesn't bring up the well dimensions on a basement bedroom window, they don't know the code.

Does Modex measure windows for free?

Yes. Every estimate includes a free in-home measurement visit. I take the measurements, check the frame for rot or settling, confirm whether each opening is insert-friendly or full-frame, and write up a line-item scope with lump-sum pricing per window. Call 952-206-6339 or request a free estimate online.

Ready for real numbers? Call Modern Exterior Systems at 952-206-6339. Free measurement, honest line-item quotes, no pressure. We serve Eden Prairie, Minneapolis, and 90+ Twin Cities communities.

Modern Exterior Systems is a women-owned, family-operated roofing and exterior contractor based in Eden Prairie, MN, serving the Twin Cities metro since 2007. Owner Joe Dvorak brings decades of hands-on construction experience, CertainTeed ShingleMaster and Malarkey Emerald certifications, and a LIFETIME workmanship warranty to every project. BBB Accredited with an A+ rating.