How to measure

TWO NUMBERS. THAT'S ALL WE NEED.

BLOCKER is made to your recess, not your glass. A couple of quick checks, then a tape measure gets you the two figures — width and drop — that go straight into your order.

Time 5-minute measure
Tools Tape measure & pen
You'll record Width & drop (mm)
Unsure? Send us photographs

STEP 1 / 4

Before you measure

Two quick checks before the tape measure comes out.

Three nested rectangles: the outer window frame, the internal recess, and the glass. The recess is highlighted as the correct thing to measure. RECESS — MEASURE HERE

Recess, not glass

Measure the internal opening BLOCKER sits in — not the visible glass, and not the outer frame.

Check your clear depth

Measure the recess depth at its shallowest point. Most windows are fine — this only matters if a handle, lock or winder projects into the opening.

If you have a handle, lock or winder
  1. Measure the full recess depth.
  2. Measure how far the handle projects into the room.
  3. Subtract the projection from the recess depth.

70 mm recess − 28 mm handle = 42 mm usable depth

If that looks tight, send us a photograph before ordering and we'll check it with you.

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STEP 2 / 4

Measure your width & drop

This is the whole job — everything else on this page is just backup. Two measurements, taken three times each, straight into your order.

Width

Measure straight across the recess at the top, middle and bottom.

Your width Use the smallest of the three. Don't reduce it further — BLOCKER applies the fitting allowance automatically.

Drop

Measure top to bottom on the left, through the centre, and on the right.

Your drop Use the smallest of the three.
A recess opening measured at six points: three horizontal width measurements at the top, middle and bottom, and three vertical drop measurements on the left, centre and right. TOP MID BTM L C R WIDTH — TOP / MID / BTM DROP — LEFT / CENTRE / RIGHT
FIG. M2 — Six points, one opening. Black lines are your width; green lines are your drop.

At a glance

The same two checks, drawn out.

A window recess with the width measured at the top, middle and bottom, and the height measured at the left, centre and right. LEFT CENTRE RIGHT TOP MIDDLE BOTTOM Order size = smallest width × smallest height
Width × 3, height × 3 Measure the inside of the recess, in millimetres. Openings are rarely square, so we build to the smallest figure in each direction.
A cross-section through a window recess showing the full depth, a handle projecting into the opening, and the usable clear depth that remains. HANDLE FULL RECESS DEPTH USABLE CLEAR DEPTH Full depth − handle projection = usable depth
Clear recess depth Measure from the front edge of the recess back to the nearest obstruction. If a handle, latch or winder projects into that space, subtract its projection.

STEP 3 / 4

Check the window can still operate

For inward-opening and tilt-and-turn windows, check the proposed frame position won't obstruct handles, hinges or the moving sash. If unsure, send us photographs before ordering.

STEP 4 / 4

Final double-check

Tick each one off as you go.

Worked example

What your width and drop should look like

Width (mm)

Top1200
Middle1197
Bottom1199
Order width1197 mm

Drop (mm)

Left1400
Centre1398
Right1401
Order drop1398 mm
Got your width and drop? Enter them straight into the configurator — BLOCKER calculates your price and fitting allowance automatically.
Start your BLOCKER

Common mistakes

FIVE WAYS A MEASUREMENT GOES WRONG

Measuring the glass

BLOCKER is fitted to the recess, so glass size alone is not enough.

Only measuring once

Older and plastered openings are often not perfectly square.

Ignoring the handle

A deep recess can still have too little usable depth if a handle projects into the fitting zone.

Rounding too aggressively

Record the measurement you actually read.

Making your own deduction

Do not subtract an arbitrary 5–10 mm. Enter the exact dimensions and let BLOCKER apply the fitting allowance.

Tape not level

A sagging or angled tape reads long. Keep it flat against the surface.

Not confident?

NEED US TO CHECK?

Send the photos and figures below and we'll check your measurements before your order goes into production.

Send us

  1. One straight-on photograph showing the full window
  2. One side-angle photograph showing recess depth
  3. A close-up of handles and locks
  4. Your width and drop measurements
  5. Your clear recess depth