Total length is the easy half. The materials list comes from post spacing, and post spacing is decided by the corners and the gate rather than by dividing the run into equal pieces.
Do not measure the whole boundary as one number. A fence is a series of straight runs meeting at corners, and each run is planned independently because each one starts and ends with a post.
Walk the line and sketch it, marking every corner, every change of direction, and where the gates will go. Then measure each run from corner to corner. A laser measure handles this well provided there is something at the far corner to shoot, and a stake with a piece of white board on it solves that for a few dollars.
Where the ground slopes, measure the horizontal distance rather than following the slope, because posts go in vertically and their spacing is horizontal. On a steep run this is a real difference.
Once you have the run lengths, the post count follows from the spacing your panel or rail system requires.
Panel fencing is rigid: panels come in a fixed width, so posts sit at exactly that spacing and the run length has to accommodate whole panels plus post widths. If your run does not divide evenly, you cut a panel or adjust the run, and it is better to know that at the planning stage.
Rail and picket fencing is flexible: you choose a spacing within the limits of the rail spans, so you can distribute the run evenly and avoid an awkward short bay at one end.
Either way, count posts as runs plus one. A 40 foot run with posts every 8 feet needs 5 bays and 6 posts. Every corner is a post shared between two runs, so count it once, not twice.
A gate needs a post at each side and those posts are usually heavier than the line posts, because they carry a swinging load. Decide gate positions and widths first, then lay out the remaining posts around them. Retro-fitting a gate into a finished run means pulling posts.
Gate openings are measured between the inside faces of the gate posts, not between post centres, and the gate itself is slightly narrower than the opening to allow for hinges and clearance.
Size the opening for what actually needs to get through. A pedestrian gate is one thing. A gate that has to admit a lawnmower, a wheelbarrow, a trailer or a delivery is another, and it is worth measuring the widest thing that will ever need to pass rather than the thing you are thinking about today.
For a double gate, remember that the two leaves meet in the middle and each needs clearance, so the opening is not simply twice a single gate.
Fence height is usually set by what you are allowed rather than what you want. Local rules commonly limit front boundary height more tightly than rear, and there may be conditions attached to fences on a corner plot for visibility.
Measure the height you intend from the finished ground level, and remember that posts need to go into the ground as well as above it. A common working figure is that a post is set to roughly a third of its above-ground height, but this varies with soil, exposure and fence type, so check the specification for your system.
On sloping ground you also choose between stepping the fence, where each bay is level and the fence steps down, or racking it, where the panels follow the slope. Stepping needs longer posts at the low end. Racking needs a system that permits it. Measure the fall along the run so you know which you are dealing with. Measuring a height difference across a slope is covered here.
Check the boundary. A fence built on the wrong line is expensive to move and creates a dispute with a neighbour that outlasts the fence. Your own measurements establish the length, not the legal position. The land guide covers this distinction.
Also check for services before digging post holes. Buried cables and pipes are frequently shallower than expected, and a post hole is exactly deep enough to find them.
Still working out which laser measure to buy? The front page narrows it to eight.