How to Use This Calculator
- 1
Enter your TDEE
Input your Total Daily Energy Expenditure (maintenance calories) in kilocalories. This is the number of calories you burn per day to maintain your current weight.
- 2
Select your gender
Choose Male (600 kcal fast-day limit) or Female (500 kcal fast-day limit) per NHS 5:2 guidelines.
- 3
Set your diet duration
Enter how many weeks you plan to follow the 5:2 protocol. The calculator will project your estimated fat loss over that period.
- 4
Review your insights
The results will show your Weekly Calorie Deficit, Fast Day Allowance, Daily Average (Weekly), Weekly Calories Consumed, Estimated Fat Loss, and Total Deficit. Additionally, the 'Your 5:2 Diet Insights' panel provides a summary of your plan's effectiveness, average daily intake, and total projected fat loss.
Example Calculation
A male with a TDEE of 2,500 kcal wants to follow the 5:2 diet for 12 weeks to lose weight steadily.
Daily TDEE
2500
Gender
Male
Diet Duration
12 weeks
Results
Weekly Calorie Deficit
3,800 kcal
Fast Day Allowance
600 kcal
Daily Average (Weekly)
1,957 kcal/day
Weekly Calories Consumed
13,700 kcal
Estimated Fat Loss
13.0 lb
Total Deficit
45,600 kcal
Tips
Choose Fast Days Strategically
Schedule your two fast days on non-consecutive days with lighter activity. The 600 kcal (male) or 500 kcal (female) limit is fixed per NHS guidelines, so plan meals around protein and vegetables to maximize satiety.
Eat at Maintenance on Normal Days
On your five normal eating days, consume your full TDEE. Avoid overcompensating for fast days, as this can negate the intended caloric deficit. The weekly deficit depends on maintaining normal-day intake at maintenance level.
Monitor Your Weekly Deficit
A weekly deficit of 3,000 to 4,000 kcal (typical for many on 5:2) equates to roughly 1 lb of fat loss per week. If the deficit feels too aggressive, slightly increasing your TDEE estimate or choosing the lower fast-day limit can help maintain energy.
The 5:2 Diet Calorie Calculator helps you plan your intermittent fasting approach by calculating fast-day allowances, weekly calorie deficits, and estimated fat loss over time.
Based on NHS 5:2 guidelines, it uses fixed fast-day limits of 600 kcal for males and 500 kcal for females, combined with your maintenance calories (TDEE) and desired diet duration to project realistic outcomes.
Understanding the 5:2 Diet Structure
The 5:2 diet restricts calories on two non-consecutive days each week while maintaining normal intake for the remaining five days.
Unlike percentage-based approaches, the NHS 5:2 protocol sets fixed fast-day limits: 600 kcal for men and 500 kcal for women.
On the five normal days, you eat at your full TDEE (Total Daily Energy Expenditure).
This creates a predictable weekly calorie deficit that drives gradual fat loss without requiring daily calorie tracking on normal days.
The Calculation Logic
This calculator uses three inputs to determine your 5:2 diet outcomes:
Fast Day Allowance = 600 kcal (male) or 500 kcal (female)
Weekly Calories Consumed = (TDEE x 5) + (Fast Day Allowance x 2)
Weekly Calorie Deficit = (TDEE x 7) - Weekly Calories Consumed
Daily Average (Weekly) = Weekly Calories Consumed / 7
Estimated Fat Loss = (Weekly Deficit x Weeks) / 3500
The weekly deficit represents the difference between what you would eat at maintenance every day versus what you actually consume on the 5:2 plan.
Fat loss is estimated using the standard approximation of 3,500 kcal per pound of body fat.
Worked Example: Male, TDEE 2,500 kcal, 12 Weeks
Consider a male with a TDEE of 2,500 kcal who plans to follow the 5:2 diet for 12 weeks:
- Fast Day Allowance: Per NHS guidelines for males:
600 kcal - Weekly Calories Consumed: Normal days plus fast days:
(2,500 x 5) + (600 x 2) = 12,500 + 1,200 = 13,700 kcal - Weekly Calorie Deficit: Maintenance minus actual intake:
(2,500 x 7) - 13,700 = 17,500 - 13,700 = 3,800 kcal - Daily Average (Weekly): Total weekly divided by 7:
13,700 / 7 = 1,957 kcal/day - Total Deficit over 12 weeks:
3,800 x 12 = 45,600 kcal - Estimated Fat Loss:
45,600 / 3,500 = 13.0 lb (5.9 kg)
The calculator reports: Weekly Calorie Deficit of 3,800 kcal, Fast Day Allowance of 600 kcal, Daily Average of 1,957 kcal/day, Weekly Calories Consumed of 13,700 kcal, Estimated Fat Loss of 13.0 lb, and Total Deficit of 45,600 kcal across 12 weeks.
Interpreting the Deficit Assessment
The calculator provides a contextual assessment of your weekly deficit as a percentage of your maintenance calories:
- 25% or higher — Aggressive (high deficit, common with lower TDEEs)
- 15-25% — Moderate (well-suited for 5:2, most users fall here)
- 5-15% — Mild (gradual fat loss)
- Below 5% — Minimal (consider verifying your TDEE)
For the example above, the deficit percentage is 3,800 / 17,500 = 21.7%, which falls in the "Moderate" range and is well-suited for the 5:2 approach.
How Professionals Interpret 5:2 Diet Results
Registered dietitians and sports nutritionists look at the weekly deficit and daily average together.
A weekly deficit around 3,500-4,000 kcal typically correlates to about 1 pound of fat loss per week, which is considered sustainable.
They also check that the daily average remains above 1,200 kcal for women and 1,500 kcal for men to ensure adequate nutrition.
If the daily average drops too low (which happens with lower TDEEs), they may recommend a modified approach or medical supervision.
The fixed fast-day limits in the NHS model provide a simple, predictable structure that most people can follow consistently.
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Frequently Asked Questions
What is the 5:2 diet and how does this calculator work?
The 5:2 diet involves eating normally for five days and restricting calories on two non-consecutive fast days. This calculator uses fixed fast-day limits per NHS guidelines: 600 kcal for males and 500 kcal for females. It then calculates your weekly calorie deficit, daily average intake, and estimated fat loss based on your TDEE and chosen duration.
Why does the calculator use fixed fast-day calories instead of a percentage of TDEE?
The calculator follows NHS 5:2 guidelines which recommend a fixed 600 kcal limit for men and 500 kcal for women on fast days, rather than a percentage of TDEE. This standardized approach ensures fast days are restrictive enough to create a meaningful weekly deficit regardless of individual TDEE levels.
How is the weekly calorie deficit calculated?
The weekly deficit equals your TDEE multiplied by 7 (what you would eat without dieting) minus your actual weekly intake (TDEE times 5 normal days plus fast-day calories times 2). For example, a male with a 2,500 kcal TDEE has a weekly deficit of 17,500 minus 13,700 = 3,800 kcal.
How accurate is the estimated fat loss projection?
The calculator estimates fat loss using the standard approximation that 1 pound of body fat equals about 3,500 kcal of deficit. For a male with a 2,500 kcal TDEE over 12 weeks, the total deficit of 45,600 kcal projects roughly 13.0 lb of fat loss. Actual results vary based on metabolic adaptation, hydration, and adherence.