Honestly IDK. I have three 275 gallon tanks. 2 of them are down in my cellar and there is one in the back of a 26ft box truck that I bought 4 years ago that I use for storage parked out in my driveway. I put 300 gallons in the house and 100 in the back of the truck last May I believe. Just a day before we got 2 feet of snow up here on Dec 16th and 17th I had my oil guy top off all 3 tanks before the storm and it took 370 gallons but the price was a buck less at 439.9. It was still 1627.00. I had a couple of cords of wood laying around in my back yard that I cut a couple of years ago. It was still good so I split it by hand just to use it up because I stopped using my wood stove 3 years ago after I had to replace some of the internal pieces in it. So I used that wood off and on from the end of October to almost the end of January when it finally ran out. I just used it to heat my first floor and the loft and only turned the furnace on to heat up the hot water for the dishes and taking a shower, but now I keep the furnace on all the time. The past 3 seasons without the wood, I would fill all 3 tanks usually the end of November to the beginning of December. By the 2nd week in March I'd have a quarter tank on each tank inside, then I would back the truck up near my house and put a hose over the male brass petcock at the bottom of the tank, stick the other end into the filler tube for the tanks downstairs which is near my front porch and gravity feed in inside and it would bring it up to roughly 3/4 full on each tank. By the time the end of November rolls around I'd have a little less than a half in each tank. By this coming November I should have a lot more oil left than I did the previous 3 years because of me using the wood stove.Just out of curiosity how long does that 400 gallons last you?