Today’s not just for stiff upper lips.

Dec. 11 is the year’s most fertile day, according to stats compiled by Lovehoney, a British online seller of sex toys.

The most popular birthday in the UK is Sept. 16, according to birth records — so you do the math.

Bracing for extra business, Lovehoney co-founder Neal Slateford said a lot of couples “will be staying in and having a good time!’’