For cheques, contracts, invitations and homework — typed as digits, returned as words.
This page writes the international/American convention without “and” (the “and” is reserved for cents in cheque style). British usage inserts it — add it back if your reader expects it.
A two-digit decimal reads the everyday way: 12.34 is “twelve point thirty-four”. Anything longer or leading with a zero is read digit by digit — 3.14159 is “three point one four one five nine”, and 5.07 is “five point zero seven”. Trailing zeros you type are kept: 12.330 ends “three three zero”.
Up to ±999 quadrillion — beyond that, digits stop being exactly representable and you probably want scientific notation anyway.
Yes — 12,5 and 12.5 both work, and thousands separators are fine
(1,234.56 or 1.234,56). A lone comma followed by exactly three digits,
like 1,234, is read as one thousand two hundred thirty-four.
Entirely in your browser. Numbers you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.