The Importance of GSM and Board Weight in Rigid Mailers
Understanding GSM: the single most important specification for board-backed envelopes.
What is GSM?
GSM stands for grams per square metre and measures the weight/density of the board backing. Higher GSM means denser, more rigid board that resists bending more effectively. It is the single most important specification when choosing do not bend envelopes.
Quick answer
350-400gsm for standard documents. 500-600gsm for prints and photographs. 700gsm+ for fine art and irreplaceable originals. GSM directly determines bend resistance. Under-specifying risks damage; over-specifying wastes money and adds postage weight. Test a sample at your chosen GSM with your actual contents to verify it provides adequate protection before ordering in bulk.
GSM levels and applications
Standard business correspondence, invoices, standard certificates.
The most popular range for e-commerce print sellers.
Fine art, signed editions, irreplaceable documents.
Over-specifying wastes money and pushes you into heavier postage bands.
GSM and postage
Board weight directly affects envelope weight, which determines postage band. A C4 envelope with 400gsm board weighs roughly 80-100g (cheapest band). At 600gsm, it weighs 100-130g (next band up). At 700gsm, it may push into the 101-250g band. Calculate the total packed weight to determine which postage band applies.
Choose wisely: size and material or hardboard vs kraft.
How to choose
GSM selection0 / 3 checked
Board-backed envelopes in every GSM
From 350gsm to 700gsm+, find the right protection level at ProcuraPack.
Browse do not bend envelopesFrequently asked questions
350-400gsm for documents. 500-600gsm for prints. 700gsm+ for valuable originals.
Yes, higher GSM adds weight which can push you into more expensive postage bands.
Pack a sample with your contents and flex it. It should resist bending without excessive rigidity.
500-600gsm for print sellers. This range balances protection with postage efficiency.
Yes, use lower GSM for robust items and higher for delicate ones.
Part of our guide
Product Guide: Do Not Bend Envelopes
Expert guides on rigid mailers and board-backed envelopes for protecting documents, prints, and certificates.
Browse all guides →
