Can you mail homemade postcards? Size, thickness, stamps and the address side

Postal services do not care who printed a postcard, only that it can travel through their machines and be delivered. A card you design, print at home and cut to size is treated exactly like one bought from a souvenir shop, provided it meets the same physical rules. There is no licence, registration or special stamp needed for printing your own.

Rules differ from country to country, so treat the figures in this guide as a starting point and check your own postal service before posting a large batch. The good news is that the standard sizes a home printer produces naturally, A6 from an A4 sheet and 5.5 × 4.25 in from US Letter, sit comfortably inside the common limits.

In the United States, the US Postal Service treats a rectangular card from 3.5 × 5 in up to 4.25 × 6 in, and at least 0.007 in thick, as a postcard. Four postcards printed on a US Letter sheet come out at 5.5 × 4.25 in, which sits inside that range, so they qualify for the postcard rate as long as the card is thick enough. Anything bigger than 4.25 × 6 in is charged as a letter.

In the UK, Royal Mail's minimum size is 140 × 90 mm, and an A6 postcard of 148 × 105 mm clears it. Many other services set a similar minimum, often around 140 × 90 mm, and some reject cards that are too floppy. Card of 200–300 gsm is stiff enough almost everywhere; anything thinner than about 170 gsm risks being creased, jammed or returned.

Sorting machines read the right half of the back, so keep it clear. Put your message on the left, a vertical dividing line down the middle, and on the right a stamp box in the top corner with three or four address lines below it. Leave a margin of a centimetre or so at the bottom: in many countries the machines print a barcode there, and writing over it can delay the card.

Write the address in dark ink with a ballpoint or fine-liner, which works on matte card without smudging. Avoid glitter, raised stickers, ribbons or anything that makes the card uneven; non-machinable items can be charged extra or held back. Foldpic's postcard back already uses this layout: caption and message on the left, divider, stamp box and address lines on the right.

Buy postage at the postcard or letter rate for your country; prices change regularly, so check the post office's current rates rather than relying on an old stamp. International postcards need the international rate, and a few services also ask for a small airmail label or marking. Stick the stamp in the box on the top right of the address side.

If your card is larger than the postcard maximum, uneven, or you are worried about it arriving pristine, put it in an envelope. Then it travels as a letter and the size rules relax. Foldpic's gift kit can print a matching envelope sized to your card, which is handy for postcards sent as part of a gift or invitation.