Health Tools

Learn how a period calculator predicts your next period, fertile window, and ovulation date, what affects cycle regularity, and how to track your cycle accurately for planning or conception.

"My due date is March 15th" doesn't directly answer "when should I start my leave" — leave planning means working backward from an estimated date, applying jurisdiction-specific policies, and building in a buffer for the very real possibility of early arrival. Here's why most leave frameworks let leave begin automatically upon birth regardless of the planned date, why work-handover targets should sit several weeks before the planned leave-start date, and why partner/co-parent leave planning is inherently date-range-based rather than fixed.

The luteal phase (ovulation to menstruation) is remarkably consistent at 12-14 days — most cycle length variation happens in the follicular phase (menstruation to ovulation), which is where stress, illness, and travel affect timing. Here's why this makes period prediction harder than it appears, what a short luteal phase means for fertility, and why perimenopause breaks cycle-tracking app predictions.

A landmark study tracking 6,400 people found metabolism is remarkably stable from age 20 to 60 — what actually changes is muscle mass. Here's the Pontzer 2021 findings, how sarcopenia drives the "skinny fat" phenomenon, why protein needs increase with age, and why resistance training is the primary evidence-based intervention.

Adding up exercise calorie estimates to calculate weekly expenditure overstates the benefit by 30-50% — because NEAT drops on exercise days, appetite partially compensates, and training efficiency improves. Here's the dose-response relationship between volume and actual burn, why MET values have a ±20-30% accuracy ceiling, the long-term resting metabolism boost from added muscle (13-22 kcal/kg/day), and why fasted exercise doesn't reliably burn more fat over 24 hours.

Twin pregnancies average 36-37 weeks rather than 40 — meaning a standard due date calculation represents a date most twin pregnancies won't reach. Here's why multiples arrive earlier, how chorionicity affects monitoring and timing, why IVF dating from embryo transfer is the most precise method available, and what vanishing twin syndrome means for dating.

Learn how BMI is calculated, what the categories mean, why it has limitations for muscle, fat distribution, and ethnicity, and how to use a free BMI calculator as part of a broader health picture.

Height is more than a number — it reflects childhood nutrition, genetics, and health system quality across generations. Here's the full conversion reference, what drives global height variation, and why measured height matters for health calculations.

Approximately 50% of kidney stone patients have a recurrence within 5 years — and inadequate hydration is the single most modifiable risk factor, because concentrated urine allows crystals to nucleate before being flushed out. Here's the stone formation chemistry for each type (calcium oxalate, uric acid, struvite, cystine), why restricting dietary calcium actually increases oxalate stone risk, why 2 litres of urine output requires approximately 3 litres of fluid intake, and why lemon juice increases urinary citrate as a validated dietary intervention.

Brown adipose tissue was thought to exist only in infants until 2009, when PET/CT scans revealed metabolically active brown fat in adults — concentrated around the neck and collarbones, inversely correlated with BMI. Here's the white vs brown vs beige fat distinction (storage vs heat generation vs metabolic plasticity), how cold activates UCP1 thermogenesis adding 100-300 kcal/day, and why thyroid hormone is the primary BMR regulator that calculators completely ignore.