This is a 20 mm arbor. Please be aware that if you order this and expect it to fit your 1-in blade arbor and it doesn't work you will not be able to return it due to your mistake. Please double check this before ordering is all sales will be final. No returns will be accepted. This is the cheapest on eBay as of the time of this listing. Ships UPS or USPS based on cheapest option. eBay counter