How much blood do you lose on your period? The numbers in millilitres, and what they mean for period underwear

The numbers, per whole period

StatPearls, the clinical reference on the NCBI Bookshelf, states it directly: "A normal menstrual cycle has a frequency of 24 to 38 days and lasts 2 to 7 days, with 5 to 80 mL of blood loss." It splits that into three bands, describing the volume of flow as heavy (more than 80 mL), normal (5 to 80 mL), or light (under 5 mL). The top of normal is sixteen times the bottom, and both ends are normal.

The CDC gives the typical figure in kitchen units instead: "Usually, menstrual bleeding lasts about 4 to 5 days and the amount of blood lost is small (2 to 3 tablespoons)." A US tablespoon is about 15 mL, so that is roughly 30 to 45 mL for a whole period. That conversion is ours, not the CDC's. The Cleveland Clinic uses the same typical figure and marks the heavy side of it, describing "Losing more than 5 tablespoons of blood during your period instead of the typical 2 to 3 tablespoons."

The NHS states no millilitre figure at all. It defines heavy periods entirely by what you have to do about them: how often you change a product, whether you double up, whether you bleed through to clothes or bedding. That is not an oversight. Nobody measures their own period.

• Normal, whole period: 5 to 80 mL (StatPearls).
• Typical, whole period: 2 to 3 tablespoons, roughly 30 to 45 mL (CDC figure, our conversion).
• Heavy, whole period: more than 80 mL (StatPearls).
• Duration: 2 to 7 days (StatPearls), about 4 to 5 days (CDC). More than 7 days is a heavy sign at both the CDC and the NHS.

Why a whole period is not one day

The per-period figure is the one clinicians use, and the least useful one for choosing underwear, because you do not wear one pair for a whole period. That volume arrives across 2 to 7 days by the StatPearls range, or about 4 to 5 by the CDC, and it does not arrive evenly. None of the sources here puts a number on how much lands on the heaviest day, so we will not put one there either. What follows is the part that matters: an hourly rate worked out from a whole period understates the hour that decides whether a pair holds, because that hour is nowhere near the average. The hours guide does the per-hour arithmetic; this page stays with the per-period volume.

Turning a period into pairs

Every brand publishes a capacity claim for each of its tiers, and our absorbency table normalizes all of them to millilitres with a grade for how each claim is evidenced. The live catalog holds 502 in-stock period-underwear names across nine brands, 365 of them carrying a verified capacity claim. The largest single band is 18 to 36 mL, with 139 names topping out inside it. Take that as the moderate middle and the arithmetic is easy to do out loud.

A typical period of 30 to 45 mL comes to between about 0.8 and 2.5 moderate pairs at their claimed ceilings: 30 divided by 36 at one end, 45 divided by 18 at the other. More than 80 mL, the heavy threshold, comes to more than 2.2 to 4.4 of them, which is 80 divided by 36 and 80 divided by 18. Those are our sums on their figures, and every capacity in them is a brand's own claim rather than an independent measurement.

Read it the right way round. It does not mean two pairs cover a period. You change a pair because it has reached what it holds that day, not because a period has run out, and the claimed ceilings are laboratory maximums at full saturation. The one published test that used real blood (DeLoughery and colleagues, BMJ Sexual and Reproductive Health, 2023) measured the period underwear it tested holding under 3 mL each before saturating, with a 2 mL average across that product category. So the sums are a way to picture the volume, not a purchase plan. Buy the tier with margin.

What a full pair looks and feels like

The volume language runs out exactly where you have to decide something, so here is the physical version. A pair at capacity feels damp against you rather than dry, because the wicking top layer has stopped keeping ahead of the core. It feels heavier and sits closer. A leak at a leg opening or the waistband is the last sign, not the first.

There is no gauge, and that is the real difference from a pad: nothing to look at, nothing to count. It matters more than it sounds, because every list in the next section is built on counting soaked disposables, and there is nothing to count once you switch. The feel-wet guide covers why a dry-feeling pair can still be near full, and the leaking guide covers what a leak usually turns out to mean.

When the numbers say talk to a clinician

The authorities describe heavy bleeding in product changes rather than millilitres, for the reason above. Here are their lists, stated as they state them. If any of it sounds like your period, the next step is a clinician, not a bigger tier. We do not diagnose anything on this site and we have no threshold of our own.

• CDC, signs of heavy menstrual bleeding: needing to change your tampon or pad after less than 2 hours; a menstrual flow that soaks through one or more pads or tampons every hour for several hours in a row; needing to double up on pads to control your menstrual flow; needing to change pads or tampons during the night; menstrual periods lasting more than 7 days; a menstrual flow with blood clots the size of a quarter or larger; a heavy menstrual flow that keeps you from doing the things you would do normally; constant pain in the lower part of the stomach during your periods; being tired, lacking energy, or short of breath.
• NHS, signs of heavy periods: needing to change your pad or tampon every 1 to 2 hours, or empty your menstrual cup more often than is recommended; needing to use 2 types of period product together, such as a pad and a tampon; periods lasting more than 7 days; passing blood clots larger than about 2.5cm (the size of a 10p coin); bleeding through to your clothes or bedding; avoiding daily activities, like exercise, or taking time off work because of your periods; feeling tired or short of breath a lot.
• NHS, when to see a GP: if heavy periods are affecting your life, if you have had heavy periods for some time, if you have severe pain during your periods, if you bleed between periods or after sex, or if you have heavy periods and other symptoms such as pain when peeing, pooing or having sex.
• Cleveland Clinic, the urgency line: "Bleeding through two or more tampons or pads each hour for two to three hours in a row is a sign that you should see your provider or seek emergency care."

Sources

The facts above trace to:

• Normal loss of 5 to 80 mL per period, the more-than-80-mL heavy threshold, the light band under 5 mL, and the 2-to-7-day duration: StatPearls (NCBI Bookshelf), abnormal uterine bleeding. Read 2026-09-05.
• The typical 2 to 3 tablespoons over about 4 to 5 days, and the signs of heavy menstrual bleeding: CDC, "About Heavy Menstrual Bleeding." Read 2026-09-05.
• The heavy-period signs stated as product changes, and the see-a-GP list: NHS, "Heavy periods." Read 2026-09-05.
• More than 5 tablespoons against the typical 2 to 3, and the two-or-more-an-hour emergency sign: Cleveland Clinic, "Menorrhagia (Heavy Menstrual Bleeding)." Read 2026-09-05.
• Real-blood capacity of the period underwear tested, under 3 mL each and 2 mL as the category average: DeLoughery and colleagues, BMJ Sexual and Reproductive Health, 2023, Oregon Health & Science University. Read 2026-09-05.
• Brand tier ceilings in millilitres and the catalog band counts: our own absorbency table and the live catalog, computed 2026-09-05. Each capacity is that brand's own claim, graded for how it was sourced.
• Thinx's period line stating "up to 12 regular tampons' worth" with no millilitre figure attached: Thinx, "How They Work." Read 2026-09-05. The roughly 108 mL our table carries for that tier is our own computation from that tampon count at the top of the FDA regular-tampon absorbency range (21 CFR 801.430). Thinx does not state it.
• The tablespoon-to-millilitre conversion (a US tablespoon is about 15 mL) and the pairs arithmetic are ours, worked from the figures above.