Boiling potatoes is hardly rocket science, but it does take some care and attention to make sure you don't overcook or undercook them. For most recipes, potatoes should be fork tender – soft enough that you can stick a fork or other pointy object into them with very little effort.
There are a variety of ways to tell whether potatoes are tender from poking them with a fork or skewer to cooling them slightly and then squeezing them to observing whether their skins are splitting.
Degrees of Tenderness
You would think that all tender boiled potatoes are created more or less equal, but the fully cooked definition that is right for potato salad is different from the degree of tenderness that's right for mashed potatoes. One thing that all tender potatoes have in common is that they are completely devoid of crispiness or crunchiness. If you bite into an undercooked potato, it'll have a crunch like a radish or a carrot, although it will be less crispy the closer it is to tender. Eating undercooked potatoes is not only unappealing, it may also cause abdominal cramping because of indigestible starches that become digestible as the potatoes are fully cooked.
Potato salad: Unless you prefer your potato salad with the consistency of mashed potatoes, the potatoes should be cooked enough to be tender, but not cooked so fully that they start to break down. Check them often with a fork, skewer or tip of a knife, and drain them when they are just tender or just past the point of crunchiness.
Mashed potatoes: If you cook potatoes until they're extremely tender for making mashed potatoes you'll save yourself some elbow grease during the mashing process. Cooking potatoes until they're very tender makes mashed potatoes less lumpy because it's easier to fully mash them. You can test them with a fork or other pointy object or check them visually to see when they start to fall apart. Don't let the boiled potatoes fully break down, however, or your mashed potatoes will be soggy and watery.
Roasted Potatoes: Unless you want to roast your potatoes for over an hour, it makes sense to parboil, or quickly boil them ahead of time. Unlike potatoes for potato salad, par-cooked potatoes can be a bit crunchy because they'll continue to soften during the roasting process. Check them with a fork or other pointy utensil and when they offer little resistance, they're ready to drain.
Size Matters With Boiled Potatoes
You can boil your potatoes whole or you can cut them into pieces before boiling them. Whole potatoes will take considerably longer than potatoes cut into bite-sized pieces unless you're boiling baby potatoes, which are already bite-sized. If you cut the potatoes into 1-inch pieces, it will take about five to 10 minutes for them to become fork tender, depending on the level of heat you use. A potato the size of your fist can take more like 45 minutes to boil until tender.
When boiling potatoes, use pieces of more or less equal size to ensure more or less equal cooking and tenderness. If you're cutting them when raw, do so as uniformly as you can. If you're boiling whole potatoes, choose spuds of roughly equal size.
Best Types of Potatoes to Boil
Some potatoes boil better than others. Russets and Yukon Golds are somewhat starchy, so they can get too soft and fall part when you boil them. That's fine for mashed potatoes, but it can be unappealing in potato salad. Red potatoes, purple potatoes and fingerlings tend to hold together in cohesive pieces even once they're tender so they're a better choice for boiling.