Three fits, and only one of them is the one people measure
An appliance has to satisfy three separate geometric problems and they fail in different places. The first is the alcove: the body plus its air clearances against the width, height and depth of the hole it lives in. The second is the swing: how far the door reaches into the room when it is open far enough to be useful, which is a larger number than most people picture. The third is the route in, which is a different problem again and is governed by the narrowest point on the path rather than by the destination.
The alcove is the one everybody measures and it is the one that fails least often, because the published body dimensions are accurate and the tape does not lie. The swing is the one that ruins kitchens, because a 32 inch door leaf standing open reaches roughly 30 inches out from the face of the unit, and if the island is 40 inches away then the person loading the appliance has ten inches of floor to stand in. The route is the one that ends with a unit sitting in the hallway.
Measure the opening at the top, the middle and the bottom, and use the smallest of the three. Walls are not parallel, drywall bulges at the corner bead, and the base cabinet run is often a sixteenth or two out of square along its length. Measure the depth to the front of the counter and separately to the front of the cabinet doors, because the counter overhang is usually an inch or more and the two numbers give different answers to the question of whether the unit looks flush.
What the air clearances are for
The gaps around an appliance are not styling. A refrigerator rejects the heat it removes from its interior through a condenser, and where that condenser is depends on the model: some are on the back, some are underneath with a fan, and some are in the side walls of the cabinet. If the heat cannot get away, the compressor runs a longer duty cycle to achieve the same interior temperature, which costs electricity every day for the life of the unit and shortens that life. A range needs clearance because its sides get hot and the cabinet next to it is combustible. A front-loading washer needs it because it moves during a spin cycle and a machine touching a wall transmits that into the structure of the house.
Rear clearance is rarely about air alone. The cord has to bend somewhere without being crushed, a water supply line to an ice maker needs a radius, a drain hose needs a standpipe, and a dryer needs the exhaust duct behind it, which on a rigid elbow can be the single deepest thing in the alcove. Measure to the back of the duct or the valve box, not to the drywall.
Corners, and the doors that will not open in them
The awkward case is an appliance whose hinge side is against a wall or a tall cabinet. The door swings toward that wall as it opens, and the leaf sweeps out roughly its own thickness in that direction before the outer face clears. Two to three inches of setback is the common figure and the installation instructions state it for the specific model.
The second half of the corner problem is worse and is often missed. A door open to exactly 90 degrees does not clear the opening of the cabinet it is attached to, because the door itself is thick and it sits proud of the body. Sliding a shelf straight out, or lifting a crisper drawer clear, needs the door somewhere past 110 degrees, and a full-extension drawer in a freezer compartment can want 130. Set the swing selector on this page to what you actually need to do rather than to the number that sounds sufficient, and then look at what is left of the walkway.
The route in
The fit calculation on this page checks the narrowest way the unit will pass an opening while standing upright, which covers most appliances, because appliances are usually taller than they are wide and taller than they are deep. What it deliberately does not do is the corner-turning geometry, where the diagonal of the item matters and a unit that walks through a doorway can still be defeated by the turn at the end of a hallway. The furniture fit checker handles rotation through a right-angle turn, tilting through an opening and the stair-landing case, and it is the right tool when the route has a corner in it.
The inches that usually decide it are the ones nobody counts. A hung door slab takes about 1.75 inches out of the opening even standing wide open, and taking it off its hinges gives that back. Refrigerator doors commonly come off, taking two to four inches of depth and the handles with them, at the cost of an hour and a careful look at the wiring loom in the top hinge. Handles are frequently the single widest point on a unit and frequently removable with two screws.
One thing that is not a clearance question but ends the same way: if the unit travels on its side or its back, sealed-system refrigeration equipment wants to stand upright for a good while before it is powered up, so the oil can drain back out of places it should not be. The delivery paperwork or the manual gives the figure for the model. It is free to wait and expensive not to.
Where this stops
Gas appliances change the problem. A gas range or dryer involves a fuel connection, and a gas appliance that burns fuel in the room needs combustion air and, in many cases, a vent. That is licensed work and it is not a clearance calculation. The same applies to anything that has to be hard-wired rather than plugged in. If you are planning the space for one, get the rough-in dimensions from the installation instructions and get the connection done by someone licensed to do it.
For the rest of the planning around a new appliance, the appliance running cost calculator covers what it costs once it is in, the replacement budget calculator covers when the rest of the house is due, and the big purchase checklist covers the questions that are not arithmetic.
Questions people ask
How much bigger than the appliance should the opening be?
That figure comes from the installation instructions for the specific model and there is no universal answer, which is why this page asks you for it rather than supplying one. What is general is the direction of the risk. If the opening is snug and the unit is a hair wider than published, you are cutting a cabinet stile. If the opening is generous, you get a gap you can see from the doorway and food falls into it. Where you have the choice, err generous on the sides and tight on the depth, because side gaps are hidden by the door overhang and depth is what everybody sees.
Why does my refrigerator stick out past the counter?
Because standard-depth units are usually around 30 to 36 inches deep including the door, and a standard base cabinet with its counter is around 25 to 26. That difference is normal and it is not an installation error. Counter-depth models exist specifically to close it, at the cost of interior volume for the same footprint, which is a real trade rather than a free one. If you go that route, work out what capacity you actually need first with the refrigerator size calculator, because a counter-depth unit typically gives up several cubic feet against a standard-depth one of the same width.
Can I take the doors off to get it through?
On most refrigerators, yes, and it is the standard answer to being two or three inches short. The manual has the procedure and it usually involves the handles first, then the top hinge cover, then the wiring loom that runs through the hinge on anything with a dispenser or lights in the door. The two cautions worth stating: photograph the loom before you disconnect it, and note that the doors are heavier than they look and want a second person. If the shortfall is more than about four inches, doors will not save it and you should look at the route rather than the unit.
Does the walkway width in front actually matter, or is it just comfort?
It is the difference between a kitchen that works and one that irritates you daily, and it is not recoverable after the cabinets go in. The practical thresholds are roughly these: under 24 inches you cannot stand square in front of an open door, which is where you unload groceries; around 36 inches one person works and a second can squeeze behind; 42 inches and up is where two people use the space at once without choreography. An appliance door open into a walkway consumes about the width of its own leaf, so a 32 inch door takes a 42 inch walkway down to 12 while it is open.
What about the space above and behind for a washer and dryer?
Behind a dryer the exhaust duct is usually the deepest object and a rigid 90 degree elbow can eat five or six inches on its own, which is why periscope-style fittings exist. Behind a washer, the standpipe, the shutoff valves and the hose bends set the depth, and a recessed valve box in the wall buys back several inches. Above them, a stacked pair needs the stacking kit dimensions rather than the sum of the two heights, and a front-loader on a pedestal gains height that has to come from somewhere. The duct side of it is worth its own arithmetic, which is what the dryer vent length calculator is for.