Pensum
A calorie tracker with period tracking built in
Log your food, your weight and your period in one app. Pensum learns how your cycle moves the scale and keeps that water out of your calorie target.
The short version
- Pensum tracks your calories, your weight and your period in one free app.
- Around your period the scale usually rises by about half a kilo. It's water, and it's gone within a week.1,2
- Pensum learns your own pattern from the days you log and keeps that water out of your weight trend, your measured expenditure and your calorie target.
- After two periods it tells you when the next one is due, and it can remind you the day before.
- Your history can come straight from Health Connect. The dates stay on your phone.
If you have a period, you know the week. The scale climbs by half a kilo, sometimes more, and a few days later it drops back. None of it is fat. It's water, and your body moves it on its own schedule. A calorie tracker that adjusts your target from your weight can't tell the difference. It sees the gain, concludes you burned less than it thought, and trims your calories, usually in the week you're hungriest. Pensum needs one extra piece of information to get this right: the days you bleed.
Your food, your weight and your period in one app
Switch on Period tracking in Settings and you get the parts of a period tracker most people actually use, inside the app you already log food in.
- Mark a period day from the same sheet you log your weight in, or enter a whole period at once by its first and last day.
- The period screen shows which day of your cycle you're on and how many days until the next period is due.
- One notification the day before, if you want it.
- A plain history of past periods with your average cycle length and period length, ready to show a doctor.
- If your period app saves to Health Connect, Pensum reads your past dates in one go and writes back the days you mark, so the two never disagree.
- Your diary calendar marks period days and the days the next one is expected.
The dates stay on your phone and in the backup file you save yourself. They aren't uploaded with cloud backup, and they never reach Pensum's analytics. On a profile set to male, none of this appears anywhere.
Why your weight goes up before your period
The clearest measurement comes from a 2023 study that followed 42 women through a full cycle with bioimpedance, which separates water from tissue. Body weight rose by about 0.45 kg. All of it was extracellular water. Fat, muscle and the water inside cells didn't change, and the gain was gone within the first week of the cycle.1 A much older study, with repeated weighings in 1965, found cycle-linked swings under one kilogram in 98% of the women it followed.2
The timing comes from a year-long study of 62 women and 765 cycles, with ovulation confirmed by temperature. Fluid retention was lowest in the middle of the first half of the cycle, built up over the last days before the period, and peaked on the first day of bleeding.3 Oestrogen and progesterone levels alone didn't predict how much water a woman held, so Pensum doesn't try to model hormones. It learns your pattern from your own scale. The mechanism itself is well described: oestradiol lowers the thresholds for thirst and for the hormone that holds water back, and it expands blood plasma volume.4
Half a kilo on average, then, rising over the last few days of the cycle, highest on day one and cleared within a week. How much varies from person to person, and nobody has measured that variation well. The studies report kilograms in groups of young women weighing roughly 55 to 65 kg, and none of them looks at body size. The water involved is the water outside your cells, which makes up about a fifth to a quarter of your body weight, so a larger body plausibly shifts more of it.11 In a small comparison, women with obesity showed a rise in that water in the second half of the cycle and lean women didn't.11 That's why Pensum starts from an average scaled to your weight and then moves to your own shape as you log.
Does your period change how many calories you burn?
A little. Too little to plan around. A 2020 meta-analysis of 26 studies and 318 women found resting metabolism slightly higher in the second half of the cycle, and about half of the individual studies found no difference at all.5 Measured over whole days in a metabolic chamber, 121 women burned about 106 kcal a day more in that phase.6 A 2024 study found about 40 kcal a day, and no difference between pill weeks in women on a combined pill.7
Appetite moves the same way. Pooled across 15 datasets, women ate about 168 kcal a day more in the second half of the cycle.8 If you log what you eat, Pensum already sees that. The burn side adds up to one or two percent over a whole cycle, which is inside the margin of any expenditure estimate, whether it's measured or comes from a TDEE calculator. So Pensum never changes your calorie target by cycle phase. It corrects your weight, and your target follows.
How period water weight throws off your calorie target
Pensum measures your expenditure from what you eat and how your weight trend moves. If you ate 2,000 kcal a day for two weeks and your trend held steady, you burned about 2,000. If the trend went up, the app assumes some of that food was stored. Water breaks that assumption. One kilogram of water moving through the two-week window looks like roughly 550 kcal a day of food you didn't burn, before any smoothing. Smoothing takes most of it out, though not all of it. With your target on Auto, the leftover moves your calorie target too.
In my simulations, a typical half-kilo period shifts the measured expenditure by about ±75 kcal over each cycle. If you hold a kilo, it's about ±110. You'd see your number and your target dip after every period and creep back over the following weeks, for no reason that has anything to do with your metabolism.
How Pensum keeps the water out
The expenditure engine itself doesn't change. Pensum changes the weight it reads. Before your trend line is calculated, each weigh-in near a period has an estimate of that day's period water taken off:
weight used = scale weight − water offset(days from the nearest period start)
On most days of your cycle the offset is zero. It only applies within a week either side of a period start.
Your first period: the average
With one period logged, Pensum uses the shape from the research: a smooth curve that peaks on the first day of bleeding, rises over the 5 days before it and falls over the 3 days after.1,3 The peak is 0.75% of your body weight, the share 0.45 kg makes of a 60 kg body. That's 0.45 kg at 60 kg, 0.6 kg at 80 kg and 0.75 kg at 100 kg. The body weight it uses is your typical weight over the surrounding month, so it follows you through a cut or a bulk.
From your second period on: your own pattern
Pensum compares each day's weigh-in with your typical weight over the surrounding month, a 29-day running median. For each day from a week before to a week after a period start, it takes the median of those differences across all your logged periods. The result is your own water curve. It's blended with the average according to how many periods it has seen:
Two periods count your own curve at half, six at three quarters. If your water peaks a couple of days early or clears slowly, Pensum ends up on your curve. If you barely retain water at all, it ends up near zero. Your weight only sets where Pensum starts, and your own periods take over from there. Pensum also corrects the days before a period you haven't logged yet, using the date it expects the next one.
The results
Nobody's true daily burn is known outside a lab, so I test in simulation, the same way I test the expenditure engine. I build a body with a known, flat expenditure, add realistic daily scale noise and a period water curve the body never stores as fat, log every period day, and let Pensum work out the correction from those logged days alone. None of the settings above were tuned against these test worlds. The first three bodies weigh 80 kg; the last two test the weight scaling at both ends.
| Simulated body | Without period dates | With period dates |
|---|---|---|
| 0.5 kg of period water, 28-day cycles | ±75 kcal | ±12 kcal |
| 1.0 kg of period water, 28-day cycles | ±110 kcal | ±29 kcal |
| 0.5 kg, cycle length varying like real cycles9 | ±45 kcal | ±22 kcal |
| 110 kg body, 0.8 kg of period water | ±72 kcal | ±19 kcal |
| 52 kg body, 0.4 kg of period water | ±34 kcal | ±18 kcal |
With no cycle at all, scale noise alone produces about ±25 kcal on this measure. The corrected numbers sit at or near that floor.
Where the test is easy and where it isn't. The two bodies whose water is exactly 0.75% of their weight have the curve Pensum starts from, so those rows flatter Pensum. The one-kilo body and the irregular cycles are the real test: Pensum has to learn a curve that differs from where it started, and it still removes between half and three quarters of the swing. Whether real water scales with weight the way I assume is something no simulation can settle, which is why your own periods take over from the second one on. Some bodies won't follow a smooth curve at all. In that case your learned curve stays close to the average, and you get the average correction and nothing worse.
When is your next period due?
Pensum predicts the next period as your last start plus the median of your last six cycle lengths, and it says "around" that date. Real cycles vary. In more than 600,000 cycles from 124,648 women, the average was 29.3 days, and each woman's own cycle varied by about 2.6 days from one to the next.9 The Apple Women's Health Study, 165,668 cycles, found a median of 28 days, with most between 26 and 30, and more variation under 20 and over 45.10 No calendar method gets much closer than a couple of days, and Pensum doesn't pretend to.
A gap of ten or more days without bleeding starts a new period, so spotting mid-cycle isn't counted as one. Gaps shorter than 15 days or longer than 60 are left out of your averages, because they're usually a gap in logging. If you log nothing for 60 days, the prediction and the reminder stop quietly, and they come back with the next day you log.
What period tracking in Pensum doesn't do
The due date is there to help you plan. Pensum doesn't predict ovulation or a fertile window, doesn't estimate your chances of conceiving, and can't be used for contraception. It doesn't log symptoms, flow or mood, and it never labels a cycle as irregular or late. Those are medical questions with different rules, and a calorie tracker shouldn't answer them.
Every setting and where it comes from
| Setting | Value | Basis |
|---|---|---|
| Average water peak | 0.75% of body weight | 0.45 kg of extracellular water measured by bioimpedance,1 about 0.75% of a 60 kg body; under 1 kg in 98% of women weighed.2 Scaled to weight because that water is a fixed share of body size.11 An assumption from the mechanism: no study has measured the change against body weight. |
| Rise before / fall after | 5 / 3 days | Retention builds before the period and peaks on day one;3 cleared within the first week.1 |
| Your curve's reach | ±7 days | Wider than the average curve, so an early peak or slow clearance still fits. |
| Comparison window | 29 days | One cycle long. A 15-day window halved the correction in my tests. |
| Blend toward your curve | periods / (periods + 2) | Set in testing. The research doesn't measure person-to-person variation well enough to set it. |
| Gap that starts a new period | 10 days | The shortest common cycle is about 21 days and bleeding lasts up to 8.9 |
| Cycles counted in averages | 15 to 60 days | Outside the range real cycles fall in, with margin.9 |
| Due-date median | last 6 cycles | Recent enough to follow changes with age, long enough to ride out one odd cycle.9,10 |
| Quiet pause | 60 days | About two missed cycles. Pregnancy, menopause and a break from logging look the same to the app, and all get the same silence. |
| Calorie change by phase | none | The second-half burn increase of 40 to 110 kcal a day is offset by appetite and sits inside the estimate's margin.5,6,7,8 |
Questions
Is there a calorie tracker with period tracking built in?
Pensum is one. You log food, weight and period days in the same app. It shows when your next period is due, can remind you the day before, and uses your period dates to keep water weight out of your calorie target. It's free, with no ads and no account.
Why do I gain weight before my period?
Mostly water. Studies measure about half a kilo on average, almost all of it water held outside your cells, peaking on the first day of bleeding and gone within a week. Fat and muscle don't change. A heavier body likely holds a little more, and Pensum scales its starting estimate to your weight.
Does my period change how many calories I burn?
Slightly. Resting burn is about 40 to 110 kcal a day higher in the second half of the cycle, and appetite rises by a similar amount. Over a whole cycle the effect is one or two percent, too small to plan meals around.
I'm in a calorie deficit. Why did I gain weight on my period?
Because the gain is water, not fat. Around a period the scale usually rises by about half a kilo of water held outside your cells, highest on the first day of bleeding and gone within a week, so a deficit that was working is still working. A tracker that sets your target from your weight can mistake that water for food you stored. Pensum takes period water out of your weight trend before it measures your expenditure.
Will Pensum change my calorie target during my period?
Pensum doesn't raise or lower your target by cycle phase. It removes period water from your weight trend, which keeps your measured expenditure and your target from dipping after each period.
Can I bring my history from my period app?
If your period app saves to Health Connect, yes. Turn on Period dates in Pensum's period screen and grant access. Pensum reads your past periods in one go and writes the days you mark back to Health Connect.
Is my period data private?
The dates stay on your phone. They are left out of cloud backup and never sent to Pensum's analytics, and Pensum sells no data. The backup file you save yourself includes them, so you can move them to a new phone.
Can Pensum predict ovulation or be used for birth control?
No. Pensum predicts your next period for planning and doesn't estimate fertile days or ovulation. Don't use it for contraception or to plan a pregnancy.
Pensum is a fast, private calorie and macro tracker for Android. Period tracking is free. Get Pensum on Google Play, read how Pensum measures your expenditure, or go back to the homepage.
References
- Kanellakis S, et al. Changes in body weight and body composition during the menstrual cycle. American Journal of Human Biology, 2023;35(11):e23951. doi:10.1002/ajhb.23951. 42 women, bioimpedance: about 0.45 kg, all extracellular water, fat and lean mass unchanged, resolved within the first week of the cycle.
- Watson PE, Robinson MF. Variations in body-weight of young women during the menstrual cycle. British Journal of Nutrition, 1965;19:237-248. doi:10.1079/BJN19650023. Repeated weighings; variation under ±1.0 kg in 98% of subjects.
- White CP, Hitchcock CL, Vigna YM, Prior JC. Fluid retention over the menstrual cycle: 1-year data from the Prospective Ovulation Cohort. Obstetrics and Gynecology International, 2011;2011:138451. doi:10.1155/2011/138451. 62 women, 765 cycles, ovulation confirmed by basal temperature. Retention peaks on the first day of flow. Self-reported, not weighed.
- Stachenfeld NS. Sex hormone effects on body fluid regulation. Exercise and Sport Sciences Reviews, 2008;36(3):152-159. doi:10.1097/JES.0b013e31817be928. Oestradiol lowers osmotic thresholds for thirst and vasopressin and expands plasma volume. Total body water is only mildly affected.
- Benton MJ, Hutchins AM, Dawes JJ. Effect of menstrual cycle on resting metabolism: a systematic review and meta-analysis. PLoS ONE, 2020;15(7):e0236025. doi:10.1371/journal.pone.0236025. 26 studies, 318 women. Luteal resting metabolism higher, standardised effect 0.33 (95% CI 0.17 to 0.49). 53% of studies found no difference.
- Ferraro R, Lillioja S, Fontvieille AM, Rising R, Bogardus C, Ravussin E. Lower sedentary metabolic rate in women compared with men. Journal of Clinical Investigation, 1992;90(3):780-784. doi:10.1172/JCI115951. Respiratory chamber, 121 women. 24-hour expenditure about 106 kcal/day higher in the luteal phase after adjusting for body composition.
- Löfberg IE, et al. Resting energy expenditure, metabolic and sex hormones in two phases of the menstrual and hormonal contraceptive cycles. Medicine and Science in Sports and Exercise, 2024. jyx.jyu.fi/handle/123456789/96775. 38 naturally cycling women and 19 combined-pill users. Luteal resting expenditure +40 kcal/day; pill phases −2 kcal/day apart.
- Hazell TJ, McCarthy SF, Bornath DPD, Tucker JAL, Khoja JS. The effect of the menstrual cycle on energy intake: a systematic review and meta-analysis. Nutrition Reviews, 2025;83(3):e866-e876. PMC11819481. 15 datasets, 330 women. Luteal intake about 168 kcal/day higher, with poor phase verification in most included studies. See also Rogan MM, Black KE. Dietary energy intake across the menstrual cycle: a narrative review. Nutrition Reviews, 2023;81(7):869-886. doi:10.1093/nutrit/nuac094.
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019;2:83. doi:10.1038/s41746-019-0152-7. Mean cycle 29.3 days, luteal phase 12.4 ± 2.4 days, within-woman cycle-length SD 2.6 days.
- Li H, Gibson EA, Jukic AMZ, et al. Menstrual cycle length variation by demographic characteristics from the Apple Women's Health Study. npj Digital Medicine, 2023;6:100. PMC10226714. 165,668 cycles from 12,608 participants. Median 28 days (IQR 26 to 30). Variability about 45% higher under 20 and at 45 to 49.
- Maury-Sintjago E, et al. Obese women have a high carbohydrate intake without changes in the resting metabolic rate in the luteal phase. Nutrients, 2022;14(10):1997. doi:10.3390/nu14101997. 15 lean women (median about 56 kg) and 15 with obesity (about 66 kg): the share of extracellular water rose in the luteal phase only in the obese group. Small and cross-sectional.