printf Format Tester

How to use

1. 1Paste your printf format string into the top field. The full %[flags][width][.precision][length]conversion grammar is supported.
2. 2Enter one argument per line in the second field. Numbers, true, false, and null parse as JSON values. Plain text is treated as a string. Wrap a string in double quotes if you need escapes like \n.
3. 3Read the rendered output on the left and the per-token breakdown on the right. Each conversion spec is explained with flags, width, precision, and length modifiers.
4. 4If the expected argument count and the supplied count differ, the page surfaces a warning so you can fix the format string or add the missing argument.
5. 5Click any Copy button to grab the full output, an individual rendered segment, or a ready-to-paste C printf call.

About this tool

printf Format Tester parses a C or POSIX printf format string, applies the arguments you provide, and shows the rendered output next to a per-token breakdown that explains what each conversion specification does. The parser walks the full %[flags][width][.precision][length]conversion grammar, recognises every flag (- + space # 0 '), accepts literal or asterisk-driven width and precision, and reads the standard length modifiers (hh, h, l, ll, j, z, t, L) without changing the rendered value for that token. It supports the conversion characters that real format strings actually use: %d and %i for signed decimals, %u for unsigned decimals, %o for octal, %x and %X for hexadecimal, %b for binary (a glibc, Python, and Go extension), %f and %F for fixed-point floats, %e and %E for scientific notation, %g and %G for the compact form, %a and %A for hex floats, %c for characters or code points, %s for strings, %p for pointer-style hex, and %% for a literal percent. Each argument line you type is parsed as a JSON literal first (numbers, true, false, null, and quoted strings work as expected) and falls back to a raw string when the line is not JSON, so most real arguments paste in without ceremony. The output panel reports the rendered text, the number of spec tokens, the expected argument count, and warnings when too few or too many arguments are supplied. The token breakdown explains every spec in plain English, surfaces the resolved width and precision, copies each rendered segment individually, and exports the whole call as a ready-to-paste C printf line so you can verify a tricky format against a real compiler in seconds. Useful for debugging logs, reproducing CSV or fixed-width report layouts, learning the printf grammar, and converting between C, Python percent formatting, and Go fmt strings without guessing. Everything runs locally in your browser, so the format string, the arguments, and the rendered output never leave your device.

Free to use. Works in your browser. No signup, no login.