What size stamp do you need for wedding invitations?

In the UK, stamps are not sized to the envelope; they are just sticky labels. What matters is the size and weight of your envelope, because that determines the postage rate. Most wedding invitations fit in a C6 envelope (114mm by 162mm) or a C5 envelope (162mm by 229mm). If your invitation is a single card in a C6 envelope and weighs under 100g, a standard first or second class stamp will usually cover it. If you use a larger envelope, a thicker card, or add multiple inserts, it may fall into the 'large letter' category, which needs a higher value stamp. The best way to know is to weigh your fully assembled invitation and measure its thickness. Post offices have a gauge that checks if your envelope is within the letter or large letter size. You can also check the Royal Mail website for current size and weight limits. Do not guess, because underpaying means the recipient may have to pay a surcharge. If you are unsure, ask at the post office counter and they will tell you the exact stamp value.