Hp cartridge refill?

hi i got a hp f2100 all in one printer and the black ink is low,can i refill it i got black ink,the cartridge no is 21

if you can refill how many mls do put in it

Yes it is possible to refill it, but go slow. Filling it too fast will cause air bubbles that will ruin the cartridge.

The hole for filling is in the middle near the blue HP logo.

i used to use refilling devices to replace ink on my epson printer, but the 2nd time i did it, it exploded in my printer, and pretty much ruined it.

Although it's a bit more expensive, i reccoment buying new cartridges and recycling the old ones.

You probably can refil it, but I wouldn't recomend it, as I used to do it, take your old Cartridges to a shop and pay them to Refil them...

Refill is ok if you are not into professional colour documents production. They don't come out fine like the original, though not usually noticed by the general public, it can ruin your chances when bidding for professional colour-related jobl.
Also limit number of refills, many cartridges' sensors will fail woefully after about three refills. The trick is, the more expensive the Inkjet printer is, the cheaper the ink cartridge. Leave refill for students and hobbyists.

I tried refilling a couple of #45 cartridges. The instructions said to just fill until it was full. Both cartridges seemed to print OK when the printer went through its alignment thing, but it still reported an error and wouldn't use the black cart. So I said F.I. and dusted off an old Laserjet 4M+. It has some feed problems, but with a "new" (old stock) toner cartridge, it prints better than the Deskjet ever did. A $25 repair kit should fix the paper jam problem.

Consider getting a laser printer, unless you really need color. There's some decent new ones for under $100, and refilling toner seems to be less iffy than refilling ink cartridges. Old HP Laserjets can be great too, if you don't mind the size and weight, and will likely outlast many new printers. Toner carts are expensive, but can be good for 1000's of pages.

If you google your model No and refill. instructions you should be directed to a site that tells you how to do it