Download and printout free measuring tool while still pregnant and keep it in a safe place. You can use a plain ruler but the circle cut outs make it much easier to 'try on various sizes.
It's also super common that your size will change from early pregnancy through to weaning going up and then down again.
Measure an hour after breastfeeding or pumping as nipples typically swell slightly and this can give you a false larger size.
Measure both nipples. Those girls are sisters, not twins, and it's entirely normal for them to be different sizes.
If you have regularly been expressing with a flange size that is too large you may find that:
A) Your nipples are temporarily larger than they otherwise would be, so you may start with a smaller flange size but find once you are using a more appropriate size, that your nipple size reduces again as they heal
B) A step down approach may be more comfortable if there is a big jump in sizes e.g. You've been using 24mm and measure 13mm. You may be more comfortable dropping to 17mm and then 13mm
Question anyone who thinks a 24mm flange and definitely a 27 mm flange "looks good". There's a very good chance it won't be a good fit and will cause pain, nipple/areola damage, swelling and hinder milk flow.
Do not, I repeat DO NOT add any extra space to your existing measurement. Yep, I know the company your purchased your pump from probably said “Add 3-5mm to your measurement to get the right flange size”. Nope, not now, not ever.

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Top tips for sizing your nips

The birth story of Revie