Royal Mail Guidelines for Do Not Bend Envelopes and Large Letter Sizing

Size rules

Large letter max: 353mm x 250mm x 25mm, up to 750g

✅

C4 (324 x 229mm) fits within large letter if total thickness stays under 25mm

C3 (458 x 324mm) exceeds large letter and ships as a small parcel

❌

1mm over any dimension reclassifies to small parcel at significantly higher cost

Quick answer

C4 and C5 board-backed envelopes qualify for Royal Mail large letter if total thickness is under 25mm and weight under 750g. Large letter rates start at 93p (2nd class) vs £3.35+ for small parcel. Board backing adds thickness, so check the packed envelope with contents inside. A C4 envelope with 600gsm board and a single print typically comes in around 8-12mm total thickness, well within the 25mm limit.

Weight bands

A C4 board-backed envelope with a single A4 print typically weighs 80-120g.

Multiple sheets or heavier board push into this band.

Balance protection needs against weight-band pricing.

Compliance tips

Stay within limits0 / 4 checked

Measure packed envelope thickness with callipers, not just the empty envelope

Weigh a packed sample to know which weight band you fall in

Account for labels, stamps, and any exterior additions to thickness

Test-post a sample to confirm Royal Mail classifies it as large letter

Cost savings: e-commerce shipping or size and material guide.

Pricing overview

Large letter 2nd class starts at 93p (0-100g) rising to approximately £2.20 (501-750g). First class is higher. Small parcel starts at £3.35. The difference is substantial, making size and weight optimisation critical for businesses sending board-backed envelopes in volume.

From ProcuraPack

Large letter compliant envelopes

Board-backed envelopes designed for Royal Mail large letter rates from ProcuraPack.

Browse do not bend envelopes

Frequently asked questions

C4 (324x229mm) and C5 (229x162mm) if total thickness is under 25mm and weight under 750g.

25mm total including envelope, board, contents, and any labels.

80-120g with a single A4 print, fitting the cheapest 0-100g band.

Yes, heavier board adds weight which can push you into higher price bands.

No, C3 (458x324mm) exceeds the 353mm length limit and ships as small parcel.