Sarah was forced to leave her gym because staff decided her outfit was "inappropriate".

US woman Sarah Villafañe was just trying to work out at her South Carolina college’s student gym.

She’d been in the same outfit to three of her classes during the day, before heading to the gym for a little sweat sesh.

The only problem? She was soon asked to leave, before her outfit was deemed “inappropriate”.

The outfit – a crop top and a pair of leggings – that was specifically designed for athletic activity.

Yeah… We’re confused too.

Sharing her experience in a post on Facebook, which has been shared over 500 times, said she was "not allowed to work out".

"When I walked into the gym they asked me to put on a different shirt. Obviously I didn't bring an extra shirt to the gym and wasn't about to wear my flannel while working out," she wrote.

"So I just said 'mhm ok' and went about my work out pretty pissed off that they even asked me to change."

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While she was doing her workout - which she says was in the very back corner of the gym - Sarah said a staff member walked up and told her to "put her shirt back on".

When she told them she was wearing a shirt, the staff member told her she needed to "wear a whole shirt".

Sarah asked to speak to the gym's manager who told her that unless she put on a shirt she would be asked to leave.

"I said 'But I HAVE A SHIRT ON'. He says 'Are you gonna put a shirt on?' And I said 'Well if this isn't a shirt...No. I'm not gonna put a shirt on."

Sarah wrote that she specifically chose the outfit for her exercise routine because it was comfortable, and said she didn't understand the gym's issue with her look.

"Why can't I work out in this outfit? Is my belly button distracting to the general 85% male demographic that your gym serves? I'm forced to leave, WHY? Honestly I'm so floored that I just got kicked out for this. DO BETTER."

Sarah later added that the gym had since told her she was kicked out because of "sanitary concerns". Too bad, she said, they never informed her of their concerns when she was being asked to change.

"They told me I need 'more coverage' and nothing more. Not to mention, their website has no written dress code anyway.

"The dress code at the front of the gym simply says 'Athletic wear must be worn'. I would say that what I'm wearing (whether you believe it's a sports bra or a crop top) is completely normal and considered athletic wear."

Commenters have been quick to defend Sarah's outfit, posting that the gym's actions were "ridiculous".

"I've seen guys at the gym wear muscle shirts that are so low that they shift around and you can see much of their sides and nipples," one woman wrote.

"If they can be like that, this should be fine....look at any advertiser for athletic wear and tell me this isn't exactly the kind of sportswear advertised. This is clearly sportswear."