Letterbox sizes: how to measure for a draught excluder
Letterboxes come in all sizes, and the outside plate doesn't tell you how big the hole through the door is. Two minutes with a tape measure tells you whether an excluder will fit. Here is what to measure and where.
The short answer: measure on the inside of the door. You need the size of the hole, whether there is flat door all round it, and how far any bolts stick out. Then check whether the letterbox is landscape or upright.
Four measurements
1. The hole, width and height
From the inside, measure the opening that goes through the door, not the outside plate. Measure the widest and tallest points.
2. Flat door around the hole
An inside excluder screws to the door around the hole, so it needs a flat surface on all four sides. Watch for panel mouldings, glass or a door handle close to the letterbox.
3. Bolts behind the letterbox
Many outside letterplates are held on by bolts or nuts that stick out on the inside. Measure how far they stand proud of the door.
4. Which way round it is
Most UK letterboxes are landscape (wider than tall). Upright, portrait letterboxes need a vertical excluder.
Magflap sizes
| MK3 Horizontal | MK3 Vertical | |
|---|---|---|
| For letterboxes that are | Landscape (wider than tall) | Upright (taller than wide) |
| Plate size | 340 × 100mm | 90 × 260mm |
| Post opening | 290 × 70mm | 65 × 220mm |
| Screw holes | 314mm apart lengthways, 80mm top to bottom | 80mm apart at the sides |
| Bolt clearance | 9mm as standard, 18mm with the additional gasket | |
| Doors | Wood, composite, metal; uPVC fitted inside | 35–70mm thick, inside or outside |
Your letterbox hole must fit inside the plate, with flat door all round it. If your hole is bigger than the plate, a Magflap won't cover it; email a photo to info@magman.uk and we'll tell you what will.
The easiest check: print the template
Our free fit template prints the Magflap plates at full size on A4. Check the 100mm scale line with a ruler, cut it out, and hold it against the inside of your door. If the hole sits inside the opening and the plate lies flat, it will fit.
Found your size?
Both Magflaps are all steel, made in Leicester, and come in four finishes.
From £41.99 · Free UK delivery · Free 30-day returns · 1-year guarantee
MK3 HorizontalMK3 VerticalQuestions people ask
Does it matter what size my outside letterplate is?
Not really. What matters is the hole through the door, measured from the inside, and the flat space around it.
My letterbox is very small. Will a Magflap still work?
Yes, as long as there is flat door around the hole for the plate to sit on. The plate covers the hole completely.
Not sure about the bolts?
If anything sticks out more than 9mm behind the letterbox, choose the additional gasket when you order. It gives 18mm of clearance.