Pete Rowswell from Langport, Somerset, was born with club foot which causes the bones in his leg to twist inwards.
The 30-year-old said he takes a "stupid" amount of pain relief just to be able to walk and the decision to have the operation was "easy".
He said he came up with the fundraising idea after one of his colleagues asked him how much a leg weighed.
"I was quite baffled. I didn't know," he said.
"He suggested we should run the competition at work 'for a bit of a laugh'.
"So we decided to raise money for the Royal National Orthopaedic Hospital in London which is where all the research is done. Without them I wouldn't be able to walk."
The operation to amputate his left leg below the knee is due to take place on 30 November.
Mr Rowswell, who holds black belts in a number of martial arts, has set up a fundraising campaign entitled Pete's getting legless to raise money for the hospital's redevelopment appeal.
He said he had persuaded his consultant to weigh the leg after the operation.
"The person that gets closest will win a decent bottle of champagne," he added.
A man who is having his leg amputated is raising money for charity in a guess the weight of the leg competition.