Remove everything possible from the intended space. If you are renovating an old bathroom, this will mean mirrors, medicine or cosmetic cabinets and any other furniture, like an undersink cupboard, and furnishings like curtains or rugs. Remove the toilet and existing sink or sink and vanity.
Measure the space with measuring tape. Record the dimensions on your grid paper and make a diagram of available space. This may simply be a floor plan, or it may also include walls with windows or radiators and their dimensions.
Include the arc, or sweep, of the door into the bathroom. An inward-swinging door is often the major space invader.
Tackle the door issue before selecting new fixtures. An outward-swinging door, either solid or bi-fold, clears a large space on the bathroom floor. If the bathroom space is wide but shallow, the railroad solution of a sliding door may make entering and leaving easier without taking up floor space.
Choose a new toilet, based on your measurements. In an existing bathroom, location of the outlet pipe, part of what is referred to as the carpenter's rough-in, will determine the models from which you can choose. Expect the rough-in outlet to measure 10, 12 or 14 inches from the backing wall. Some toilet models will fit all those spaces, but most work best in only one. Using an offset cabinet flange gives you a small amount of flexibility in placing your new toilet, essentially 2 inches in any direction. If you are creating a new bathroom, consider a wall-mounted toilet, which reduces the amount of floorspace needed by the fixture.
Investigate the full range of sinks and vanities available for small spaces. Ordinary vanities tend to measure approximately 26-by-26 inches; small vanities may measure as little as 14-by-14 inches. Wall-hung vanities come in both small and large sizes. At times, having the small amount of extra floor space makes movement easier, conveying a greater sense of space overall.
Locate the sink/vanity as far from the toilet as you can to make your space seem larger. This may mean moving the sink off what plumbers often call the "wet wall," which contains all the bathroom pipes and so often accounts for the line of tub, toilet and sink along a single bathroom wall even in a large room. Both small-space sinks and vanities come in corner-fitting models, which may save further space.
Store as many bathroom necessities as possible in locations outside the bathroom. Unless you have a sliding-door medicine cabinet, replace the existing cabinet with a mirror and move medication to a safe drawer or cupboard shelf. Put towels in a cupboard or dresser close to but outside of the bathroom. Hang towel bars or a flat basket for necessities on the door. This frees a small vanity for toilet paper and cleaning supplies.
Bring window treatments into space-saving mode. If you are renovating heavily, look into frosted glass or thin acrylic blocks for a privacy-protecting window. Replace curtains with a ruffled valance above closeable folding shutters to regulate available daylight.