Fiber and fibre are the same word and cover the same meanings: a thread-like strand, the indigestible part of plant food, and inner strength. Fiber is standard American spelling, fibre is standard British, Canadian, Australian and Irish spelling — for example, whole grains give you fiber in the US and fibre in the UK.
The core difference
They are identical in meaning; the spelling depends on which English you're writing.
- fiber — American spelling for a thin thread-like strand, dietary roughage, or moral resolve: Whole grains and vegetables are good sources of fiber.
- fibre — British and Commonwealth spelling of the same word: Doctors recommend more fibre in your diet.
How to tell them apart
Both spellings trace back to French fibre, from Latin fibra. Unlike the -er/-re split seen in words like theater and center, this is really just a shared French-derived spelling that American English later changed while British English kept the original form.
The confusion tends to show up in food labelling and health writing that crosses between American and British sources, since both spellings are common in nutrition content online. As with other -er/-re pairs, the rule is to match the variety of English the rest of the piece is written in and keep it consistent, including in related words such as fiberglass versus fibreglass.
