How to Measure Windows for Shutters: A 2026 Guide
Table of Contents
Tools Needed for Measuring Window Shutters
Inside Mount vs Outside Mount Shutters
How to Measure Windows for Shutters: Step-by-Step
Measuring Window Width
Measuring Window Height
Measuring for Bay Windows
Accounting for Window Obstructions and Clearance
Measuring handle projection properly
Clearance for panel movement
Motorised and smart shutter clearance
Checking the reveal is clear end to end
Troubleshooting Non-Square Windows for Shutters
Checking squareness accurately
Measuring a non-square opening
Shimming and packing
When to use a wider frame or mid-rail
When to stop and get a professional measure
Common Measuring Mistakes for Window Shutters
Conclusion
Frequently Asked Questions
Last Updated: September 23, 2026
Tools Needed for Measuring Window Shutters
Getting the right measurements starts with the right kit. You don't need much, but what you use matters more than most guides admit.
Here's what to gather before you begin:
Steel tape measure (not fabric or retractable plastic, which stretch and sag)
Laser measure for tall or awkward windows where a tape is hard to hold
Pencil and paper for noting every figure as you go
Step ladder for high windows
Torch for checking recess depth and dark reveals
Spirit level for checking squareness
A steel tape measure is the single most important tool here.
Pro Tip Measure in millimetres, not centimetres or inches. Shutter manufacturers work to tight tolerances, and a 5mm error is the difference between a clean fit and a visible gap. Write every number down in millimetres to avoid conversion mistakes later.
At Caledonia Shutters, our fitting teams measure in millimetres as standard. Now, on to the decision that trips up more people than any other.
Inside Mount vs Outside Mount Shutters
Inside mount shutters sit within the window recess. Outside mount shutters fix to the wall or architrave around the opening. The choice changes how you measure, so decide before you pick up a tape.
Feature | Inside Mount | Outside Mount |
Fits within recess | Yes | No |
Hides uneven frames | No | Yes |
Needs deep recess | Yes | No |
Best for older properties | Sometimes | Often |
Measures to | Recess opening | Outer frame or wall |
How to Measure Windows for Shutters: Step-by-Step
Measuring for shutters comes down to three figures: width, drop, and depth. Take each one at multiple points, because windows are rarely perfectly square.
Here's the full process, step by step.
Measuring Window Width
Measure the width in three places: top, middle, and bottom of the window opening. Record all three.
For inside mount, use the smallest of the three figures. Shutters must fit the tightest point.
For outside mount, use the largest figure, then add your overlap on each side.
Measuring Window Height
Measure the drop in three places: left, centre, and right. Again, note every figure.
For inside mount, use the smallest height. This is your vertical measurement.
For outside mount, use the largest, then add your overlap top and bottom.
Measuring for Bay Windows
Bay windows need each panel measured separately. Never assume the sections are identical.
Measure every panel on its own
Note the angle of each corner
Check that sill depth is consistent across all sections
Watch Out The most common bay window error is measuring one section and copying it across. Bay windows are built at angles, so each panel differs. Copying figures leads to panels that don't meet cleanly at the corners, and that gap can't be fixed on site.
Accounting for Window Obstructions and Clearance
Shutters need room to open, close, and tilt. Anything in the way will cause problems, so check for obstructions before you finalise your measurements. This is the step most DIY guides rush, and it is where made-to-measure orders most often go wrong.
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Look for these common blockers:
Window handles that project into the recess
Sill overhangs that limit where panels can sit
Radiators below the window
Curtain tracks or poles already fitted
Alarm sensors on the frame
Trickle vents built into the window head
Air bricks or vents in the reveal
Measuring handle projection properly
Measure handle projection specifically. Close the window, then measure from the face of the reveal to the furthest point of the handle. If a handle sticks out 40mm into the recess, your inside mount frame needs at least that much depth plus a few millimetres of working clearance, or you need a cut-out in the frame.
Clearance for panel movement
Clearance also applies to the panels themselves. Louvre panels need space to tilt, and hinged panels need room to swing open. As a working rule, allow:
Hinged panels: enough depth for the panel thickness plus a few millimetres so the leading edge clears the reveal when open
Bi-fold panels: clearance at the fold point, which is often the tightest part of the run
Track systems: the track depth plus the panel depth, measured from the mounting face
Motorised and smart shutter clearance
This is the detail most measuring guides skip entirely. If you are planning motorised or smart-integrated shutters, the headrail needs more room than a manual equivalent. The motor, wiring route, and battery pack (if not mains-powered) all sit within or behind the headrail, so the recess depth requirement increases.
Before you measure, confirm:
Whether the motor is mains-powered or battery-powered
Where the cable or charging point will sit
Whether the headrail needs a service gap for battery access
Whether the panels are louvre-tilt only or also slide
Pro Tip If you are unsure whether you will motorise later, measure as though you will. Adding depth allowance now costs nothing; retrofitting a motor into a shallow recess often means remaking the frame.
Checking the reveal is clear end to end
Run your tape along the full depth of the reveal at the top, middle, and bottom. Reveals are often shallower at one end than the other, especially in older properties. The usable depth is the smallest figure you find, not the average. If the shallowest point cannot take the frame plus clearance, the mount type needs to change.
Troubleshooting Non-Square Windows for Shutters
Most windows are slightly out of square, and older properties are worse. The trick is not to fix the window, but to measure so the shutters hide the problem. This is the section most guides leave out, and it is where a made-to-measure order either succeeds or fails.
Checking squareness accurately
Start by checking squareness. Measure both diagonals of the opening, corner to corner. If they differ, the window is out of square. Note the difference in millimetres.
As a working guide:
Under 5mm difference: an inside mount with a frame will usually absorb it
5-15mm difference: inside mount is possible with a wider frame or a mid-rail, but outside mount is safer
Over 15mm difference: outside mount is the honest choice, because the shutters can be set level and plumb independent of the frame
Measuring a non-square opening
For an inside mount on an out-of-square opening, you still measure width at top, middle, and bottom, and drop at left, centre, and right. But you also need to decide which figure the frame will be built to.
Shimming and packing
Where an inside mount frame meets an uneven reveal, fitters use shims or packing behind the frame to bring it level and plumb. This is normal practice and not a sign of a bad window. The important thing is to measure the reveal depth at the points where the frame will actually sit, because shimming eats into the usable depth.
When to use a wider frame or mid-rail
A mid-rail or wider frame can mask uneven reveals. A mid-rail splits the panel horizontally and gives the eye a straight line to follow, which disguises a slightly tapered opening. A wider frame does the same at the edges. Both add cost and reduce the glass area slightly, so they are a trade-off rather than a free fix.
When to stop and get a professional measure
If the diagonal difference is large, the reveal is inconsistent, or you are dealing with a bay or arched opening, the risk of an expensive remake rises sharply. This is the point where a professional measuring visit pays for itself. A fitter can check squareness, depth, and obstructions in one visit and recommend the mount type that will actually work.
Key Takeaway Non-square windows aren't a dealbreaker. They just change which mount you choose and how much frame cover you need. Measure the diagonals first, note the difference in millimetres, and let that figure decide the approach.
Common Measuring Mistakes for Window Shutters
Measuring once. Always take three readings per dimension.
Using a fabric tape. It stretches, and your figures drift.
Forgetting the frame. Inside mount measurements go to the recess, not the glass.
Ignoring obstructions. Handles, sills, and sensors all eat into clearance.
Rounding up. Never round. Use the exact millimetre figure.
Mixing units. Stick to millimetres throughout.
Watch Out The costliest mistake is ordering from a single measurement. If your width reading is 10mm too wide, the shutters won't fit the recess at all. That's a full remake, not a tweak. Measure three times, then measure once more before you order.
If measuring an awkward or non-square window feels risky, that's exactly where professional measuring pays for itself.
Conclusion
Measuring for shutters rewards patience. Three readings per dimension, millimetres throughout, and a clear decision on inside versus outside mount will get you most of the way there.
Frequently Asked Questions
How do I measure my windows for window shutters?
To measure for shutters, start by deciding on inside or outside mount. For width, measure the window opening at the top, middle, and bottom, then use the smallest measurement. For height, measure the left, centre, and right, again using the smallest. Always use a steel tape measure and note dimensions in millimetres. This ensures your made-to-measure shutters fit correctly.
Should shutters be the same size as windows?
Shutters should match the window opening for an inside mount, but for an outside mount, they need to be larger to overlap the frame. Inside mount shutters sit within the recess, so their dimensions match the opening. Outside mount shutters cover the frame and architrave, so you measure the overall area you want to cover. Always specify your mounting choice before measuring.
What is the difference between inside and outside mount shutters?
Inside mount shutters fit within the window recess, providing a clean, built-in look. Outside mount shutters attach to the wall or architrave, covering the entire window and frame. They are ideal for windows with obstructions like handles or for adding extra insulation. Your choice affects how you measure.
How do I account for window obstructions when measuring?
Check for handles, vents, or uneven reveals that could block shutter panels. For inside mounts, ensure the recess is deep enough to clear the handle projection. If not, consider an outside mount or a frame extension. Measure the clearance from the window frame to the front of any obstruction. This prevents panels from hitting the handle when opened or closed.
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