match the shape to your brewer first, then the size. cone-shaped brewers take cone filters, flat-bottom brewers take basket filters, and the size number has to fit your specific model. when in doubt, the brewer's manual or the maker's website names the exact filter.
using the wrong filter is not a disaster. it mostly means slow draining, overflow, or grounds in the cup. but the right one is cheap and easy to find once you know what you are looking for.
start with the shape
look at the bottom of your brew basket or dripper. a pointed bottom takes a cone filter. a flat bottom with drain holes takes a flat-bottom filter, sometimes called a basket filter.
some cone brewers take a pointed cone, like the ones made for the v60. others take a wider flat-v shape, the style melitta made common. they look similar but are not interchangeable. the shape of the brewer tells you which one it wants.
common size names
filter sizes are named by number, and the numbers are not consistent across brands. the same number means different things for different shapes.
for pointed cone filters, the common sizes are 01, 02 and 03. size 02 is the most common and covers most one-to-four-cup brewers.
for flat-v cone filters, #2 fits smaller brewers and #4 fits most standard drip machines. #4 is the one most people need for a regular home coffee maker.
flat-bottom brewers and automatic machines mostly use a standard basket filter for eight-to-twelve-cup machines. smaller machines take a junior size.
specialty brewers often want their own filter. flat-bottom drippers like the kalita wave have two sizes made only for that brewer, and chemex brewers use their own folded filters. check before substituting.
what if i bought the wrong size?
filters that are too big can be trimmed at the edges to fit, or folded down at the rim. go slowly so the seam stays intact.
filters that are too small can work in a pinch if you use less coffee and pour gently. overfilling a small filter is how grounds end up in the cup.
wrong shape is fixable too. a cone filter can be folded flatter for a flat-bottom brewer, and a basket filter can be nudged into a cone. the cup will be fine, it is just not the long-term answer.
how do i know for sure?
check the manual or the maker's product page for your exact brewer model. most list the filter shape and size by name. once you know it, write it on the box you keep under the counter so the next refill is easy.