Bulking means eating above your maintenance calories to build muscle. Cutting means eating below them to lose fat. That is the entire distinction, and everything else is detail about how far above or below you sit and how long you stay there.
Bulking
Muscle is expensive to build. Your body needs a surplus of energy and a reason to spend it, which is where the training comes in. Without the training, a surplus is just weight gain.
Keep protein at 1.6 to 2.2 grams per kilogram of body weight. Train each muscle group two or three times a week and add weight or reps over time, because progressive overload is the actual signal for growth. The surplus itself can be small: 200 to 350 calories a day is enough for most people, and going higher mostly buys fat.
That last point is where bulks go wrong. A 1,000 calorie surplus does not build muscle three times faster than a 300 calorie one. Muscle accrues at a rate your body sets, and the extra food goes where extra food goes.
Cutting
Cutting is a deficit with a job to do: lose fat and keep the muscle you spent months building.
Protein does most of that protecting, so keep it high, higher than during a bulk if anything. Keep lifting heavy. If you cut your training weights the moment calories drop, you tell your body the muscle is no longer needed, and it will act on that.
A deficit of 250 to 500 calories a day suits most people. Bigger deficits work faster and cost more: worse sessions, worse sleep, more muscle lost along the way. Do not go under 1,200 calories a day without medical supervision.
The real difference between them
They look like mirror images and they are not.
Fat loss is fast and reliable. Run a deficit and it happens. Muscle gain is slow and conditional, and a good year of training might add a few kilograms of it. That asymmetry should shape your planning. A cut can be measured in weeks. A bulk that only lasts a month has barely started.
Your hunger behaves differently too. Appetite fights a deficit and mostly cooperates with a surplus, which is why cutting requires more attention day to day even though bulking requires more patience overall.
Which one first
If you carry a lot of fat, cut first. A leaner starting point means a later bulk puts on proportionally more muscle, and you will not be hiding progress under a layer you plan to remove anyway.
If you are already lean and weak, eat. Trying to get visibly muscular by cutting further is a common and miserable mistake, because there is nothing underneath to reveal yet.
If you are somewhere in between and fairly new to training, you may not need to choose. Beginners often add muscle and lose fat at the same time near maintenance, which our guide to body recomposition covers properly.
Mistakes that cost people months
Guessing the starting number. Both phases depend on knowing your maintenance calories, and an estimate that is 400 calories out turns an intended lean bulk into a fat one. Work out your TDEE first, then adjust from what the scale actually does.
Switching too often. Two weeks of bulking followed by two weeks of cutting is just maintenance with extra admin. Give a phase two or three months.
Adding cardio to a bulk until the surplus disappears, or cutting all cardio during a cut and losing your conditioning. Both directions need some.
Judging progress from one weigh-in. Water shifts of a kilogram or more are routine, especially when carbs move. Look at weekly averages.
Ending a phase properly
Do not jump straight from a deficit to a big surplus. Coming out of a cut, walk your calories back up over a few weeks so your body and your appetite have a chance to settle. Our piece on reverse dieting covers how to do it, and why a maintenance phase matters makes the case for spending time in the middle before you start the next push.