If you have the goal of getting six-pack abs, one of the most important things you can do to achieve this is to take a very close look at your diet program. If you aren't eating properly, it's going to be very hard to get down to the lower levels that are required for six pack abs, thus full results will be unlikely.
Often, the standard lower calorie diet will get you down to a level of body fat where you can see the top two abs fairly clearly, but it won't be enough to get you the full six-pack abs that you're really looking for. To obtain that level of definition, you have to step up your diet game and really push hard for maximum results.
Usually, this will come in the form of carbohydrate cycling. Since protein requirements really cannot be lowered as you progress along with the diet (they in fact should be increased if anything), the only two options you have are to decrease fats or decrease carbohydrates. Typically fats are already low themselves, so this leaves carbohydrates.
Let's go over the most common carb cycling approach used, which is where you will place higher carbohydrate days on your heaviest training days and then lower carb days on off/low-intensity training days.
Figuring Out Weekly Calorie Balance
Because the calories you consume over the course of each day during the week is going to vary when you carbohydrate cycle, you should be instead using the weekly calorie balance as an indication of how many you need for weight loss.
When aiming to get down to the very low levels of body fat required for six-pack abs, expect to go down to about eight or nine calories per pound of body weight. Some individuals won't need to go quite this low, but for most it will be a requirement.
So what you'll do is take your current bodyweight in pounds and multiply by nine (always start at the highest point and work your way down as you need).
Let's say right now you're 150 pounds. This gives you an average intake of 1350 calories per day (150 X 9), or 9450 calories per week.
Figuring Out Your Daily Calorie Balance
Now that you have your weekly calorie requirement, you can use this to formulate your daily calorie balance. For best results you'll want to make your high carb days quite high and your low carb days very low.
This is to really trick the metabolism into thinking you aren't dieting at all on your higher carbohydrate days to prevent it from slowing down, as well as to provide the fuel the body needs when exercising at such a high intensity.
All of this said, you do have protein requirements you must meet. Let's set your protein requirements at 1.5 grams per pound of body weight. This is a very good number to use when really aiming to reach maximum leanness since the additional protein will both spare lean muscle mass and help keep you feeling satisfied.
So you have 150 pounds times 1.5 grams of protein per day, which equals 225 grams of protein. This then works out to an even 900 calories (225 X 4 calories/gram of protein).
Low Carbohydrate Days
On your low carbohydrate days, you're going to limit yourself to eating mostly protein with a very small amount of dietary fat as well as incidental carbohydrates—those found in vegetables or the carbs that come along with your protein source (such as with cottage cheese for instance).
So we take your protein requirements of 900 calories per day and add roughly 200 more calories to this, for a total daily intake of 1100 calories.
Be aware that these days aren't exactly going to be easy, but you're going to get great results if you can push through them - plus your non-diet days will feel that much better.
Higher Carbohydrate Days
Next, the number of calories you have on your higher carbohydrate days will depend on how many days of training you're planning.
On a diet such as this, a full body workout program tends to work quite well because it will hit the muscles with a maximum frequency (which is good from a lean muscle retention viewpoint) but will still provide plenty of time to rest to prevent overtraining on your reduced calorie intake.
You can choose an upper and lower body split instead, but be aware that this will mean more high carbohydrate days with fewer low carbohydrate days. That will not influence your weekly calorie balance since we've already pre-determined that, but it will mean lower calories on each training day.
Let's use a full body workout performed three days a week as an example. So out of your total weekly calorie requirements (9450), you have used up 4400 calories (1100 calories X the four days you aren't lifting weights). This leaves you with a total of 5050 calories to spread out over those three training days (about 1680 calories).
Since we aren't restricting calories nearly as much on these days, you can decrease your protein intake slightly to 1.2 grams per pound on those days, meaning 720 calories will be devoted to protein (1.2 X 150 pounds X 4 cals per gram of protein).
Now you have 980 calories left over. Some of this should be directed towards healthy fat since it is important for overall health, but you do want to keep fat on the lower side since our main objective is to shock the body with all the carbs to stimulate the metabolic rate, restore muscle glycogen, and provide you with more energy.
Reserve approximately 30 grams of fat (bodyweight in pounds X 0.2), so 270 calories worth. Now you have 690 calories left over for carbohydrates (1700 - 720 - 270)—or about 173 grams.
You'll want to focus the majority of these calories right around the training period since this is when your body needs them the most. Plus by doing so, you'll find that the rest of the day stays quite similar in terms of meal planning to your low carbohydrate days.
So there you have all the calculations you need to put carbohydrate cycling into full effect. Keep in mind that you may wish to vary these numbers based on your own needs, but this is meant to illustrate to you the overall concept and how you would go about setting it up.
Some individuals also like to work medium carbohydrate days in there as well (used on upper body training days so high carb days are left for lower body training days), but this will make it a bit more complicated since you'll have more numbers to work with.
Either way, with time and dedication, this type of diet when combined with a solid workout program should get you the results you're looking for.