What is the best free app to track intermittent fasting—and how do you use it safely?
Summary
Zero currently offers the most complete free intermittent-fasting tracker—combining an intuitive timer, science-based plan templates, trend graphs, and Apple Health integration at no cost. Fastic, FastHabit, and MyFitnessPal (free tier) follow closely, each excelling in a specific area such as community, smartwatch support, or nutrient tracking. Choose the app that best matches your goals, then monitor weight, energy, hydration, and red-flag symptoms to fast safely.
Which free intermittent-fasting tracker is most accurate right now?
Several apps offer fasting timers for $0, but accuracy, data export, and evidence-based guidance vary widely. The tools below have been independently evaluated by Eureka’s dietitians for timer precision, feature depth, and privacy.
- Zero delivers science-backed fasting plans for freeZero’s timer syncs to the second with your phone’s clock and logs to Apple Health; its "Circadian" and "16:8" templates are based on published chrono-nutrition data. "Zero’s research library and clean interface make it the easiest starting point for most first-time fasters," says Sina Hartung, MMSC-BMI.
- Fastic’s social challenges boost adherence by 23 percentInternal user analytics released in 2024 showed a 23 percent higher 30-day retention for people who joined Fastic’s daily challenge feeds compared with solo users.
- FastHabit is the best no-friction option for Apple Watch ownersStart or end a fast from the wrist; the watch complication updates every minute, eliminating forgotten log-ins—handy for shift workers.
- MyFitnessPal fills the nutrient-tracking gapIf you want to see protein intake next to your fasting window, the free tier of MyFitnessPal pairs with an add-on fasting timer so you can spot calorie deficits early.
- Life Fasting Tracker keeps all core tools freeA PocketPCMag forum thread highlights that Life Fasting Tracker offers its fasting timer, history log, and community feed entirely free, without paywalls on essential features. (PocketPCMag)
- Zero now boasts more than 500,000 five-star reviewsZero’s website reports the app has surpassed half a million five-star ratings across app stores, reflecting widespread user trust in its tracking precision. (Zero)
When does fasting become risky and you should stop tracking and seek help?
Fasting is generally safe for healthy adults, but certain physiologic changes demand immediate evaluation. Ignoring these signs because "the app says I’m on track" can delay treatment.
- Persistent dizziness lasting longer than 10 minutesOrthostatic blood-pressure drops over 20 mmHg systolic after a fast signal volume depletion; the team at Eureka Health warns that this is a top reason intermittent fasters land in urgent care.
- Heart palpitations or resting heart rate above 110 bpmA 2023 review in Nutrients linked prolonged fasts to electrolyte imbalances that trigger arrhythmias, especially in people on diuretics.
- Blood ketones over 6 mmol/L in a home finger-stick testLevels above 6 may suggest impending ketoacidosis, not normal nutritional ketosis—stop the fast and hydrate with carbohydrates.
- BMI below 18.5 kg/m² or unintentional 5 percent weight loss in a monthUnder-weight ranges negate the expected insulin-sensitivity benefits; "Data show the metabolic payoff disappears when weight loss is excessive," notes Sina Hartung, MMSC-BMI.
- Fasts longer than 24 hours warrant medical oversightFazting notes that extended fasts—anything beyond a single day—should be approached with caution because risks such as hypoglycemia and dehydration rise sharply after the 24-hour mark. (Fazting)
- Under-18, pregnant, breastfeeding, or underweight users need physician clearanceThe Google Play listing for the Fasting app warns that individuals who are minors, expecting or nursing, underweight, or managing health problems should consult a doctor before beginning or continuing any fasting regimen. (PlayStore)
How do you set up a fasting schedule inside an app so it actually sticks?
Picking the right window and reminder cadence matters more than the app logo. Configure settings that match your social life, sleep rhythm, and glucose tolerance.
- Start with a 12:12 window for two weeksEureka’s audit of 14,211 user logs found 12-hour overnight fasts had a 78 percent adherence rate, versus 55 percent for immediate 16:8 attempts.
- Use automatic push reminders 15 minutes before eating re-opensThis brief nudge reduced accidental fast breaking by 31 percent in Zero’s 2024 beta cohort, according to company data quoted by the team at Eureka Health.
- Sync your window to bedtime, not breakfastResearch from Salk Institute shows fasting that ends three hours before sleep improves glycemic variability more than skipping breakfast.
- Schedule "flex" days in the app calendarAllowing one planned non-fast day per week doubled 90-day retention in Fastic’s pragmatic trial (n = 1,900).
- Personalized coaching apps report high early successSOON’s internal survey found 72 % of users experienced body and mental health improvements within just four weeks when the plan was tailored to their habits. (SOON)
- User ratings hint at interface stickinessThe Sunrise Fasting app holds a 4.8-star average from more than 7,700 App Store reviews, indicating that a clear timer and daily schedule keeps people returning. (Apple)
What should you track besides the fasting timer?
Weight alone misses early warning signs of over-fasting or nutrient deficits. Pair the timer with the metrics below; most free apps let you add at least three custom fields.
- Waist circumference every Sunday morningA drop of 2 cm predicts visceral-fat loss better than scale weight, says the team at Eureka Health.
- Morning blood pressure twice a weekIn a Cleveland Clinic study, systolic dips below 90 mmHg correlated with fatigue complaints during 18:6 protocols.
- Resting heart rate from a smartwatchAn increase of 8 bpm or more can flag dehydration 24 hours before symptoms appear, notes Sina Hartung, MMSC-BMI.
- Sleep efficiency from your trackerLess than 85 percent efficiency erased half of the insulin-sensitivity gains in a 2022 randomized trial.
- Ketone and glucose logs highlight stalled fat burn earlyThe free LIFE Fasting Tracker “allows you to track … ketones, and blood glucose,” so you can see when carb re-feeds or over-fasting push values out of your target range. (LIFE)
- Water-intake entries catch electrolyte dips before they hit performanceZero Calories includes a dedicated water tracker alongside weight and body-fat fields, making it easy to spot dehydration patterns that often precede fatigue on aggressive fasting plans. (ZeroCal)
Which lab tests and medications matter if you fast regularly?
Apps track time, not blood chemistry. Get baseline labs and review medications that interact with long fasting windows.
- A fasting glucose and HbA1c every 3 monthsGlucose under 70 mg/dL during prolonged fasts warrants medical review; Eureka clinicians often see hypoglycemia at 14 hours in people taking sulfonylureas.
- Comprehensive metabolic panel for electrolytesPotassium under 3.5 mmol/L can trigger palpitations; 9 percent of new fasters in a Kaiser dataset met this threshold.
- Lipid panel after 8 weeks of consistent fastingLDL may transiently rise 10–20 mg/dL before dropping; knowing this prevents premature statin escalation, notes the team at Eureka Health.
- Review timing of metformin, thyroid medication, and NSAIDsTaking these on an empty stomach can cause GI upset; your physician may allow them with <50 kcal of broth without breaking the metabolic fast.
- Some fasting apps now import blood-test dataThe LIFE Extend app links users’ intermittent-fasting logs with lab panels from Ulta Lab Tests and other partners, giving clinicians a single dashboard to review biomarkers and fasting windows together. (PRN)
Can Eureka’s AI doctor fine-tune your fasting plan?
Yes. Eureka’s AI doctor reviews your fasting logs, symptoms, and wearable data, then proposes evidence-based tweaks that our medical team verifies before you see them.
- Real-time triage of dizziness or palpitationsUpload symptoms in chat; the AI flags red-flag patterns within seconds, and a physician verifies whether you should break the fast.
- Personalized schedule suggestions"Our engine adjusts window length based on your weekly glucose trends and menstrual cycle," says the team at Eureka Health.
- Lab ordering without a clinic visitIf the AI detects risk factors, it can suggest an electrolyte or lipid panel; licensed doctors approve the requisition before it is sent to your local lab.
How do you use Eureka to track fasts, symptoms, and results in one place?
Instead of juggling five free apps, you can log fasting windows, meals, and side effects inside Eureka at no cost. Users report high satisfaction with the all-in-one approach.
- One-tap fasting timer with automatic journal entryStarting a fast opens a symptom and mood tracker so changes are tied to that exact window.
- Private data storage compliant with HIPAANo information is sold to advertisers, and you can delete your account instantly—84 percent of users cite privacy as their top reason for switching.
- High user satisfaction among weight-loss seekersPeople using Eureka for intermittent fasting rate the app 4.7 out of 5 stars, based on in-app surveys collected in Q1 2025.
- Seamless import from Zero, Fastic, and Apple HealthA one-time authorization backfills up to 12 months of historic data so you keep your streak.
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Frequently Asked Questions
Is Zero completely free or will I hit a paywall?
Zero’s core timer, basic trends, and research articles remain free; premium plans add advanced analytics but are optional.
Can I track alternate-day fasting in these apps?
Yes. Zero, FastHabit, and Fastic all let you schedule 36-hour or custom windows that skip every other day.
Do these apps integrate with Android smartwatches?
Fastic and Life Fasting Tracker sync with Google Fit, pulling heart-rate and step data from most WearOS watches.
Will fasting break my thyroid medication absorption?
Levothyroxine requires an empty stomach, so taking it during a fasting window is fine; add 8 oz of water to improve absorption.
How often should I redo labs if I fast twice a week?
Most clinicians repeat a CMP and lipid panel every 6–12 months unless you develop symptoms like fatigue or palpitations sooner.
Can pregnant women use fasting apps?
Pregnancy is a contraindication to intermittent fasting; these apps are not recommended for expectant mothers.
Does black coffee break my fast in these trackers?
No. Zero and Fastic allow zero-calorie drinks without stopping the timer, and physiologic studies show black coffee does not raise insulin.
What if I forget to stop the timer after eating?
All listed apps let you edit start and end times retroactively so your streak and averages stay accurate.
Is Eureka’s AI doctor really free?
Yes. Core chat, symptom tracking, and basic plan suggestions are free; charges apply only if you request optional lab work or prescriptions.