I have a new polebarn 270ft from my house. I need to run 100amp 220 volt service to it do that I can power a 2 post vehicle lift, some lighting & general 120volt outlets. I plan to breaker out of my houses 200amp panel with 100 amp breaker. Run cable in conduit to a 100 amp service panel in the barn. My question is what size conduit, cable runs do I need & what size cable? I live in Michigan. Thanks for help!

SlapTop,
If you intend to use copper cable (my preference) then
Based on an 80% load at 270 ft - #1 AWG THHN/THWN copper will keep the voltage drop below 3%. So 1-1/4" PVC conduit with (3)-#1 THHN/THWN copper with a #6 AWG copper ground cable.
In addition I would suggest you think about a junction or pull box approx. halfway....it would make the pull a bit easier. Try to stay at least 18" below grade with the conduit.

Hey thanks for the response. I will go with this plan. I now plan to use copper rather than conduit. I will put it lower than 24 inches down which is code in MI as I am putting water in too which needs to be below the frost line. A friend of mine gave me a good idea also. At about 1 foot depth put caution tape so if someone starts digging they find the tape & hopefully realize the is power below. Thanks again!

Slaptop, he means using copper conductors IN conduit, not rather than conduit.

Also, a pull box half way is not what I would consider logical. It would be crazy to have to deal with this box in the middle of a yard. Also, it is much more work to do this than to simply do it in one pull.

Sorry I miss spoke I plan to use buriable copper wire so I won't need conduit.

IMO you'd be crazy to go all that distance with direct burial.
PVC is cheap insurance.