What's inside the card
The front shows how many days old they are on the day you choose, usually their next birthday, so the number is right when they open it. Inside are their heartbeats, blinks, laughs, hours asleep, full moons and kilometres travelled around the Sun, their age on Mars, Jupiter and Mercury, and a star whose light set off around the time they were born. The facing page is for your message: type one in, or leave it blank and write by hand.
Body figures use published averages that change with age, so a baby's fast heartbeat is counted differently from an adult's. They're good estimates for a typical person rather than a measurement of one.
Printing and folding
Choose "Print or save as PDF" and you'll get two A4 landscape pages: the outside (back and front) and the inside. Print them double-sided on thin card or heavy paper, set scaling to actual size or 100%, and choose "flip on short edge" so the inside lines up the right way up. No double-sided printer? Print the first page, put the sheet back in the tray blank side up, then print the second.
Trim along the small corner marks to get a 10 × 7 inch sheet, then fold it in half down the middle. It makes a 5 × 7 inch card that fits a standard 133 × 184 mm envelope.
Questions
Is it free?
Yes. Make as many cards as you like.
Do you keep the details I enter?
No. The card is built in your own browser and nothing you type is sent anywhere. When you make one for someone else, their details aren't saved at all, not even on your device.
Can I make a bigger version to frame?
Yes, the birthday poster is the same idea at A4 or A3, ready to frame.
What if they were born on 29 February?
In years without a 29 February, the card counts to 1 March.