Weight cap. of 3/4 inch plywood

It is really about balance. one 2 x 4 can take the weight but you want to distribute it evenly.

You didn't say what the foundation is. If this is a stand that will sit on the ground, I would use brick or concrete block, that way you don't have to worry about wood rot, water or insect damage.

No need to use mortar, although digging out a bit of a depression and putting down several inches of sand under the masonry would be a nice touch.

Agree

What he's described is way more than min to support it.

Maybe under-thinking it ? I'm thinking that the base - under the support - is more critical. Rain barrels will over-flow big time - during the heavy downpours. .. make sure the ground beneath the barrel is stable ! then - a big sidewalk paver - with a couple concrete blocks on top - will cost almost nothing in dollars or time - and provide a nice raised surface - get your rain barrel up high enough to get a pail under the drain tap. .. and a bonus - no rotting wood to attract bugs ! or fall apart in a few years. John T.

He may be thinking that the barrel up off the ground will gravity feed for irrigation, hand watering, eliminating a pump. I have this with my 150 gal aerator tank. It will flow enough water to fill a toilet tank with the power off but you are not taking a shower. I went with 4 concrete posts (6" pvc full of concrete) and PT 2x6 stringers and decking. I think plywood will just de laminate.

You need to use the Sagulator.

It will calculate the sag of more species of wood than I knew existed based on total load, load distribution, support method and a few more factors.

It crossed my mind too but I don't see anything sagging. The barrel is self supporting around the circumference and the supports will be directly below it.

Yeah, but where's the fun in *not* using the Sagulator? ;-)