Look at the weight of the block of butter cut it in half and you have half the weight. Cut it in half and you half the weight again (1/4 origional weight) keep doing this until you have approx 15 grams or 3 x 5grams approx. reciepes are just guidelines you can adjust quantities to suit your tastes so you don't need to be 100% accurate.
Billy... if you have a tape measure and cold butter, it's a little block that measures approx 30mm x 25mm x 20mm... well a block that shape weighed 15g in my scales! If it's melted butter, then it's a tablespoonful, plus a little bit ! (I melted the same block and poured it into my measuring spoons and it filled the 15ml / tablespoon with a little bit, about a half teaspoonful, left over). I'd go with enfable's idea about cutting a block of cold butter down and go from there.
LOL.. Billy - but you wrote *butter* in your original question and since most butter comes in blocks.... we all assumed you were using butter in a block !
I have just measured out 3 level teaspoonfuls of Utterly Butterly and that weighed 15g.
Note, this is a measured teaspoon, not an ordinary one from cutlery drawer. I'd need 3 heaped cutlery drawer teaspoonfuls of Utterly Butterly to give me 15g.
LOL Billy.... I must disagree - butter is a totally different *critter* to marg.... different source material, different taste, different uses. The only common factor to me is that both can be used on toast, but so can peanut butter, nutella, jam, marmalade hehe.. I would never put marg on a jacket potato or on vegetables or use it for frying food.