If you want bigger muscles then you will need to increase your calories, or ‘bulk‘.
For many people bulking is seen as a good excuse to eat loads and the inevitable increase in body fat is just something that comes with it.
But when done right, you can increase calories without getting too fat (though some body fat gain is likely) and use those extra calories to increase the size of your muscles.
Of course many of you have already resigned yourself to the fact that you will gain body fat during your bulking period. “It happens to everyone right so what's the problem? I'll be going into a cutting period straight afterwards” but if you didn't gain as much body fat during the bulk, then you wouldn't need to cut for as long.
Anyone who has experienced a cut knows that if there is any way to make it less painful then it should be followed. Because cutting is one of the most difficult things a Bodybuilder can do.
So how can you Bulk without getting fat?
The first thing you need to do is re-think how you begin bulking.
Most lifters just start eating more, this will not do. You need to start measuring everything. Get a tape measure and take your waist, hip, chest, arm, leg, and neck measurements. Then weigh yourself and take front & side photos of yourself so that you have an idea of where you started.
You will also need to know how many calories are required to maintain your current weight, do this through calorie tracking apps, and measuring a week's worth of calorie totals.
Another measurement you need to be aware of is your Non Exercise Activity Thermogenesis (NEAT) which is a measurement of the amount of calories you burn daily doing everything but exercising.
So walking, washing up, vacuuming, standing, running for the bus etc … This is difficult to measure exactly, but a step counting app gives you a rough idea (which is all you need really).
Once you have all of these measurements recorded you can begin making changes.
You are looking to bulk up so you will want to increase you daily calories, but doing so in a slow and steady manner is preferable to the traditional eat everything all the time approach.
Increasing your calories on a weekly or bi-weekly basis by around 50-100 calories is advisable.
You can use progress photos, measurements, and scale weight to decide each week if you are gaining too much body fat, or not gaining enough muscle, and adjust your calories.
As you can see, this is a much more professional approach to bulking and will prevent you from gaining too much body fat as you will be gauging progress each week.
Keeping your protein levels high is also a good idea, now that you are on a bulk you will be exercising at a higher intensity. This means that you will require more protein to fuel protein synthesis in the muscles .
Changing the ratio from high carb to high protein will also help improve body composition as a 2003 study by Layman et al found .
So now you've got your calories and macros worked out, and you have a handle on analysing the changes in your body you can look at exercise.
When bulking it is important to take full advantage of the calories coming in otherwise you are just wasting them (and allowing them to turn into body fat).
Use drop-sets, triple drop sets, eccentric training, super-sets, and generally increase the volume of training so that you can take advantage of the extra calories.
You will not be able to train like this when you are trying to cut, so do it now. Bulking is an excuse to lift heavy (if that's your goal) or to train to muscle failure, or both. Take this opportunity, and work hard.
Make sure that you rest effectively well though, as training to failure too often without sufficient rest can lead to overtraining (no matter how much you eat) .
Now that you are exercising more you may find that your NEAT goes down, this is because you are fatigued.
Letting your NEAT go down can actually lead to increased body fat so it is important that you keep NEAT in check.
One way to do this is to track your steps and make sure that you are hitting the same target day in day out.
Lots of studies find 10,000 steps per day to be the ideal number , but if you are already exercising daily then you can get away with less than that. Just make sure that you hit your step count on your rest days.
The traditional way to keep body fat low during a bulk is to add in lots of Low Intensity Steady State (LISS) cardio. This will still help, as will HIIT but it won't make as much difference as you expect.
Just think of cardio as the last 10%. Keeping your diet in check whilst bulking is the most important, measuring (and adjusting) is also crucial, lifting too. Then cardio is a final step.
 Tarnopolsky, M., Atkinson, S., MacDougall, J., Chesley, A., Phillips, S., Schwarcz, H. 1992. Evaluation of protein requirements for trained strength athletes. Journal of Applied Physiology 73(5): 1986-95
 Layman, D., Boileau, R, Erickson, D., Painter, J., Shiue, H., Sather, C., Christou, D. 2003. A reduced ratio of carbohydrate to protein improves body composition and blood lipid profiles during weight loss in adult women. Journal of Nutrition 133(2): 411-7
 Willardson, J. 2007. The Application of Training to Failure in Periodized Multiple-Set Resistance Exercise Programs. The Journal of Strength & Conditioning Research 21(2): 628-31
 Patrick L. Schneider, David R. Bassett Jr, Dixie L. Thompson, Nicolaas P. Pronk, and Kenneth M. Bielak (2006) Effects of a 10,000 Steps per Day Goal in Overweight Adults. American Journal of Health Promotion: November/December 2006, Vol. 21, No. 2, pp. 85-89.