Getting accurate window measurements before ordering curtains or blinds is one of the most important steps in any window furnishing project. A measurement that is off by even a few centimetres can result in blinds that do not sit flush in the recess, curtains that finish awkwardly above the floor, or treatments that fail to cover the window properly when privacy or light control is the priority.
This guide walks through exactly how to measure windows for curtains and blinds, covering both inside and outside mount installations, the specific numbers you need for each product type, and the common mistakes that lead to ill-fitting results.
Tools You Will Need
Before you start measuring, have a steel tape measure on hand rather than a fabric or plastic measuring tape. Steel tapes hold straight and give more accurate readings over longer drops. You will also need a pencil or pen for noting measurements, and a notepad or phone to record results room by room. A step ladder is useful for windows with high sills or deep reveals.
Measure in millimetres wherever possible. Millimetres leave less room for rounding errors than centimetres and are the unit most curtain and blind suppliers and installers work in. When in doubt, always measure twice before recording a final figure.
Understanding Inside Mount vs Outside Mount
The first decision that shapes how you measure is whether the curtain or blind will be installed inside the window recess or outside it.
An inside mount installation fits the product within the window recess, sitting flush with or recessed into the frame. This approach is common for roller blinds, roman blinds, and venetian blinds where a clean, built-in look is the goal. The available width and height within the recess determines the maximum product size.
An outside mount installation positions the product on the wall above or around the window frame, covering the recess entirely. This is the standard approach for curtains and is also used for blinds when the window recess is too shallow for an inside mount, or when the goal is to make the window appear larger. Outside mount measurements are taken across the wall area you want the product to cover, not just the window opening itself.
Most window furnishing projects in Australian homes use outside mount for curtains and inside mount for blinds, though this varies by window type, room, and the specific effect you are trying to achieve.
How to Measure for Roller Blinds and Roman Blinds (Inside Mount)
For a blind installed inside the window recess, you need two measurements: the width of the recess and the drop from the top of the recess to the sill.
To measure the width, take three readings across the recess: one at the top, one in the middle, and one at the bottom. Window frames are rarely perfectly parallel, particularly in older homes, so these readings will sometimes vary. Use the smallest of the three measurements as your final width figure. This ensures the blind will fit inside the recess without binding against the frame.
Most roller blind suppliers will apply a deduction to the width you provide to allow the blind to operate freely inside the recess. This deduction is typically between 5 and 15 millimetres depending on the supplier and the mechanism used. Confirm whether the deduction is applied automatically by the supplier or whether you need to apply it yourself before ordering.
To measure the drop, measure from the top of the recess to the window sill. Again, take readings at both the left and right sides of the recess and use the shorter figure. For inside mount blinds, the drop is usually fixed by the recess height, so there is limited scope for adjustment.
How to Measure for Curtains (Outside Mount)
Measuring for curtains involves more variables than measuring for blinds, because decisions about hang height, curtain width, and floor clearance all affect the final numbers.
Start with the hang height. As a general principle, curtains look best when hung as close to the ceiling line as possible rather than at the window architrave. Ceiling-height curtains make rooms feel taller, windows appear larger, and the overall result more polished. Decide on your hanging point first, whether that is the ceiling, a point 10 to 15 centimetres below the ceiling, or just above the architrave, and measure down from that point to determine your required drop.
For the drop measurement, measure from the bottom of the curtain rod or track (not the top) to the floor. Subtract 10 to 15 millimetres from this figure if you want the curtain to skim the floor cleanly. Add 30 to 50 millimetres if you prefer a slight puddle finish. The rod or track should be installed before final drop measurements are taken where possible, to avoid discrepancies between the planned and actual hanging position.
For the width, measure the full width of the wall area you want the curtains to cover when closed. This should typically extend beyond the window frame on each side, by at least 15 to 20 centimetres per side, to allow the curtains to stack clear of the glass when open and to frame the window generously when closed. Multiply this total coverage width by 1.5 to 2 to account for the fullness needed in the curtain fabric. A fullness of 1.5 times is standard for a contemporary, relaxed drape. Double fullness gives a more gathered, luxurious result.
As an example: if the area to be covered measures 1800 millimetres wide, and you want double fullness, your total fabric width requirement is 3600 millimetres, typically divided across two panels of 1800 millimetres each.
How to Measure for Venetian Blinds and Vertical Blinds
Venetian blinds are almost always installed as an inside mount, so the measurement approach follows the same process as for roller blinds. Measure the recess width at three points and use the smallest figure. Measure the drop from the top of the recess to the sill. Check with your supplier whether they apply a width deduction, as this varies by blind type and brand.
Vertical blinds are more commonly installed as an outside mount, positioned above the window recess and extending beyond it on each side. Measure the full width of the area to be covered, adding at least 50 millimetres per side beyond the window frame to ensure adequate light block. For the drop, measure from the intended top of the headrail to the floor, again accounting for the clearance you prefer at the bottom.
How to Measure for Plantation Shutters
Plantation shutters are typically installed as an inside mount, fitted directly into the window recess as a frame that sits flush with or slightly proud of the surrounding wall. Measuring for shutters follows the same basic principle as for inside mount blinds: width at three points (top, middle, bottom) using the narrowest figure, and drop at left and right sides using the shorter figure.
However, shutter measurements carry less tolerance for error than most other window furnishing products because shutters are made to precise dimensions and cannot be adjusted after manufacture. For plantation shutters in particular, a professional measure is strongly recommended even for homeowners who are comfortable measuring for other product types. The cost of an incorrectly made shutter panel far exceeds the cost of a professional measure.
If a window has an unusually shaped recess, angled reveals, or an out-of-square frame, these factors need to be identified and communicated clearly to the supplier before an order is placed.
Measuring for Layered Window Treatments
When combining a blind and a curtain on the same window, the measurements for each product need to account for the other. The blind is typically installed first, inside the recess, and the curtain rod or track is then positioned on the wall in front of it. The rod or track needs to project far enough from the wall to clear the blind mechanism when the curtain is open, typically a minimum of 80 to 100 millimetres from the wall surface.
This projection also affects where the curtain rod sits relative to the wall, which in turn affects the total width the curtain needs to cover. When measuring for a layered treatment, take note of the blind bracket depth and confirm the curtain hardware has sufficient clearance before finalising any measurements or orders.
Common Measuring Mistakes to Avoid
Measuring only once is the most common source of errors. Rooms are rarely perfectly square, and a single measurement taken at one point can miss variations in frame width or floor level that affect the finished result. Always take multiple readings and record them carefully.
Confusing the window glass measurement with the window frame or recess measurement is another frequent mistake. For inside mount products, the recess width matters, not the glass size. For outside mount products, the coverage width on the wall matters, not the frame or glass size. These are meaningfully different numbers and should not be used interchangeably.
Forgetting to account for the rod or track installation point before measuring the curtain drop is a mistake that leads to curtains arriving either too long or too short. The drop measurement must be taken from the actual hanging point, which means the hardware should ideally be in place before the final measurement is confirmed.
Finally, relying on builder’s plans or architectural drawings rather than physical measurements of the finished window is a risk in new builds. Windows and frames do not always land at precisely the dimensions shown on plans, and measuring the actual installed window rather than the planned one is always the right approach.
When to Call in a Professional Measurer
For straightforward windows with standard square frames, measuring your own windows is entirely achievable with a steel tape measure and careful attention to the steps above. For anything more complex, including out-of-square frames, bay windows, angled or raked ceilings, unusually large drops, or plantation shutters, a professional measure removes the risk of a costly error and ensures the finished product fits exactly as it should.
At Peninsula Curtains & Blinds, we offer a free measure and quote service across the Mornington Peninsula. Our team measures accurately for curtains, blinds, shutters, and layered treatments of all types, and can advise on product options, mounting approaches, and hardware choices at the same visit. Get in touch to arrange a time that suits you.