Two measurements, one piece of trigonometry
Levelling is a right triangle. You measure a height difference over a known horizontal span, and the slope in degrees is the arctangent of one divided by the other. Everything else follows from that. If you measured with a 24 inch spirit level and found half an inch of drop, and the wheels on your coach are 96 inches apart, the lift the low wheels need is half an inch scaled by 96 over 24, which is two inches. The angle is the same either way: the arctangent of 0.5 over 24, and of 2 over 96, are both 1.19 degrees.
This is why the calculator asks for the span separately from the track width. People measure with whatever they have — a level on the floor, a phone app on the counter, the difference between two tyre contact points — and the number that matters is the lift under the wheel, not the reading on the tool. Getting the span wrong is the most common way to build a stack that is half the height it should be.
The front-to-back case works identically but the correction is usually applied at one point rather than under wheels: the tongue jack on a trailer, or the front jacks on a motorhome. The lift needed there is the measured difference scaled to the distance between the axle and the jack.
Small angles you can feel
| Side to side | Across a 96 in track | What people report |
|---|---|---|
| 0.25 degrees | 0.42 in | Nothing, unless a marble is involved |
| 0.5 degrees | 0.84 in | Water pools at one end of the shower tray |
| 1 degree | 1.68 in | Noticeable lying down, doors start to have a preference |
| 2 degrees | 3.35 in | Obvious walking around, drawers roll open |
| 3 degrees | 5.03 in | Unpleasant, and a big stack of blocks |
Two things come out of this table. The first is that the angles at issue are tiny, which is why eyeballing a site is unreliable and why a two dollar bubble level stuck to the A-frame is one of the better purchases in this hobby. The second is that side to side is felt far more than front to back, because you sleep across the coach rather than along it and because a floor that tilts sideways is the one your inner ear objects to.
Whole blocks, and the overshoot nobody mentions
Blocks come in fixed thicknesses. If you need 2.0 inches and your blocks are 1.5, you get 1.5 or 3.0 and neither is 2.0. The results above show that overshoot explicitly, along with the residual angle it leaves — now tilted the other way. In practice this is fine, because a quarter of a degree in the wrong direction is better than a degree in the right one, but it is worth knowing that a stack of whole blocks lands near the target rather than on it.
If you drive up a stepped stack rather than jacking onto a flat pad, the block count is not the number of layers. A three layer ramp needs one block on top, two under it and three under those, which is six blocks per wheel, and two wheels on the low side makes twelve. That is the difference between the calculator telling you three and your box holding four.
The refrigerator, said carefully
Absorption refrigerators — the type in most RVs that run on gas — move refrigerant by gravity through a sealed cooling unit rather than by a compressor. Running them substantially off level interferes with that flow and can cause damage, which is why levelling gets discussed alongside fridges rather than only alongside comfort. What the acceptable limit is depends on the model, manufacturers publish figures for their own units, and those figures differ. Look up the number for the fridge you actually own rather than taking any general tolerance as fact, including one from a calculator page.
A useful habit regardless of the published number: parked overnight matters far more than parked for lunch, and a fridge that has been running badly off level for an afternoon is a different situation from one left that way for three days.
Getting on the blocks without getting hurt
The arithmetic is the easy half. Blocks want firm, flat ground under their whole base, because a stack on soft ground sinks unevenly and a stack on a stone rocks. The tyre needs to sit fully on top of the stack rather than hanging over an edge, which is the failure that dumps a coach off its blocks. Chocks go on before anything else is unhitched or lowered. Levelling jacks level; they are not stands and nobody should be underneath a vehicle that is resting on them. If a site needs a pile of blocks that makes you hesitate, the correct answer is almost always a different patch of ground.
Related pages
For general slope arithmetic see the slope and grade calculator and the angle and slope calculator. Hitch height and the way a trailer sits are worked on the trailer tongue weight calculator, and the weight resting on those blocks comes from the RV weight and cargo capacity calculator. If you are levelling before a long stop, the vehicle storage layup planner covers the rest of it.
Questions people ask
How many leveling blocks do I need for two inches?
It depends on the thickness of the blocks and on whether you drive up a ramp or lift onto a flat pad. Two inches of lift from one inch blocks is two layers. As a flat pad that is two blocks per wheel. As a stepped ramp you drive up, the lower layer has to extend forward for the tyre to climb, so a two layer ramp is three blocks per wheel and a three layer ramp is six. With two wheels on the low side those figures double again. The calculator shows both the layer count and the total blocks to carry, which is usually the number people are short of.
How level does an RV actually need to be?
For comfort, most people stop noticing below about half a degree side to side, which on a typical eight foot track is under an inch of difference. For an absorption refrigerator there is a manufacturer figure, it varies by model, and it is the one to look up rather than to generalise. For plumbing, the tanks drain and the shower tray sits the way the coach sits, so being level end to end mostly determines whether the tank empties fully. None of these are a single universal tolerance and anyone quoting one for all coaches and all fridges is guessing.
Should I level side to side or front to back first?
Side to side first, because it is done under the wheels and it changes as soon as the wheels move. Get the blocks placed, drive on, then chock, then unhitch or extend the jacks and correct front to back at the tongue jack or the front levellers. Doing it the other way round means the front-to-back correction changes when you drive onto the side blocks and you do it twice. On a coach with automatic levelling the system handles the sequence, though it still helps to start on ground that is close.
Can I use a phone app instead of a bubble level?
For a reading, yes, and the angles it reports are the ones this calculator wants. The catch is that a phone measures the surface it is sitting on, and the countertop, the floor and the frame are not necessarily parallel to each other. Whatever you use, use the same reference surface every time and learn what its reading is when the coach is genuinely level, because a consistent offset is easy to work around and an inconsistent reference is not. A bubble mounted permanently on the A-frame has the advantage of never moving.
Is it safe to stack blocks four or five high?
That is beyond what most stackable levelling blocks are designed for, and the practical problems arrive before the rated ones. A tall stack has a small footprint carrying a large load, it needs firm level ground under all of it, and any part of the tyre hanging over the edge concentrates that load further. Soft ground under a tall stack settles unevenly and the stack goes over. If the arithmetic on this page asks for more than a few inches, treat that as information about the site rather than as a shopping list, and move.