Key Take Aways
- Measure width first, then drop — always in millimetres.
- Inside mount blinds sit within the recess; outside mount blinds fix to the wall above and around the window.
- Inside mount: measure width at three points, use the smallest figure.
- Outside mount: add 50–75mm to each side of the frame, and 50–100mm above it for drop.
- Custom blinds are cut to order — measure twice before you submit.
Most people only measure their windows once in their life, when they’re buying blinds for the first time. Get it right and the blind fits, functions, and looks the part. Get it wrong and you’re either back to square one or living with gaps you didn’t plan for.
The process isn’t complicated. But there are two fundamentally different ways to fit a blind, and each requires different measurements. Before you touch a tape measure, you need to know which one applies to your window.
Inside Mount or Outside Mount: Decide This First
Inside mount
The blind fits within the window recess. Brackets fix to the inside of the frame. The result is a clean, flush look with the wall surface sitting around the blind like a natural border.
This works when your recess is deep enough, you need at least 40mm of clear depth for roller blind brackets, 60mm or more for venetian headrails. If the depth isn’t there, outside mount is your option.
Outside mount
The blind fixes to the wall or architrave above the window. It covers the frame and overlaps onto the surrounding wall. This suits shallow recesses, older window frames that aren’t square, and anyone who wants to make a window appear larger or block more light around the edges.
Browse the roller blind range if you’re unsure which fitting method suits your setup.
Measuring for Inside Mount Blinds
Width
Measure across the inside of the recess, top, middle, and bottom. Recess walls aren’t always parallel. Use the smallest of the three readings.
When you submit this measurement, the manufacturer applies a small deduction (usually 5–10mm) to ensure the blind clears the recess walls on both sides. You don’t need to subtract anything yourself.
Drop
Measure from the top of the recess opening to the sill. Take this at two or three points and use the smallest figure. For roller blinds specifically, stop the drop measurement just above the sill, the bottom rail needs clearance to raise cleanly without catching.
Recess depth
Measure from the glass face to the front edge of the frame. Less than 40mm and most roller blind brackets won’t fit. Less than 60mm and venetian headrails become a problem. If depth is tight, outside mount is the practical call.
Measuring for Outside Mount Blinds
Width
Start with the outer edge of the window frame, side to side. Then add 50–75mm to each side for overlap. This matters more than most people realise. A blind that barely covers the frame will have visible light gaps at the edges when closed. For a bedroom or nursery where light control is the point, use the full 75mm on each side.
Drop
Measure from where you plan to fix the headrail down to where you want the blind to end. Most people fix 50–100mm above the top of the frame, enough clearance for brackets and a small amount of overlap that stops light from coming in at the top. Drop to the sill, or 50mm below it if the window is in a room where you want proper darkness when the blind is down.
Stack clearance for roller and vertical blinds
When a roller blind is fully raised, the fabric stacks around the tube. When vertical blinds are drawn, the louvres stack to one side. If you’re fixing the headrail high specifically to keep the glass clear when the blind is open, account for the stack height in how far above the frame you mount.
Standard Blind Sizes in South Africa
Off-the-shelf blinds typically run in width increments from around 600mm up to 2,100mm, with drops between 1,200mm and 2,400mm. That covers a lot of standard windows in newer developments and suburban homes.
Older homes are a different story. Pre-1990s windows in Johannesburg, Cape Town, and Durban often have non-standard proportions, wider frames, deeper recesses, irregular heights. If your window doesn’t land on a standard increment, made-to-measure is the right route. The cost difference is usually smaller than people expect.
Measuring by Blind Type
Roller blinds
Width and drop as above, with the manufacturer applying a recess deduction for inside mount. Note whether you want the fabric to roll off the front or back of the tube, front roll sits the fabric slightly further from the glass, back roll sits it closer.
Venetian blinds — aluminium and wood
Same width and drop process. Stop the drop measurement at the window sill, venetian slats sit horizontally and there’s no benefit in dropping below the sill. The slat stack height when raised depends on slat width and total number of slats, check the product spec if headroom above the window is limited.
Vertical blinds
For sliding doors and wide windows, measure the full opening width including any door frame or fixed panel you want the blind to cover. Add recommended overlap on the non-opening side and allow stack clearance on the side where the louvres draw to when open.
The vertical blinds guide covers sliding door configurations in more detail.
Mistakes That Cost People Money
Using a fabric tape. They stretch. A reading taken with a fabric tape can be 5–10mm out. Use a steel retractable tape every time.
Measuring in centimetres. Suppliers work in millimetres. A measurement submitted in centimetres that gets misread costs you a remake. Record everything in mm.
One measurement per dimension. Windows aren’t always square, especially in older homes. Always measure at multiple points.
Skipping the recess depth check. If you’re planning an inside mount and the depth isn’t sufficient, you find out either at ordering stage or, worse, at installation. Check it upfront.
FAQ
Do I need to deduct anything from my inside mount measurements? No. Submit the actual recess width. The manufacturer applies the tolerance deduction when cutting.
What if the window isn’t square? Use the smallest measurement in each direction. If the variation is more than 10mm across the width, outside mount is usually the more forgiving option.
Can I trim a standard blind to fit? Some aluminium venetian blinds can be cut down in width. Most roller and fabric blinds can’t be trimmed cleanly without specialist equipment. Made-to-measure is the safer approach.
How do I measure a bay window? Treat each section as a separate window. Measure and order each blind independently. Contact the Blinds Online team if the bay configuration is unusual.
Get the Measurements Right and the Rest Is Easy
The actual ordering process is straightforward. It’s the measuring that determines whether your blinds land properly. Inside mount or outside mount, pick the right method for your window, record everything in millimetres, and double-check before you submit.
Order custom blinds online or request a quote if you’re working across multiple windows.