Robert Hill, aged 22, was stopped in a Fiat Punto on the A483 at Dolfor, near Newtown on May 5 this year.
A drug test found he had 6.8 microgrammes of Delta-9-Tetrahydrocannabinol per litre of blood. The legal limit it is 2mcg.
Hill, of Trefonen Avenue, Llandrindod Wells, Powys, pleaded guilty at Welshpool Magistrates Court to driving a motor vehicle with a proportion of a specified controlled drug above the specified limit.
As well as the road ban, magistrates handed him a two-year community order which includes 40 hours of unpaid work.