A ceiling of 190 bpm. That is all a 30-year-old heart can produce at maximum effort. Every training zone - fat burning, endurance, threshold, sprint - is a percentage of that number. Add a resting pulse measurement and the zones get personal: a fit runner with 55 bpm at rest gets a fat-burning zone starting at 136 bpm, while a sedentary beginner with 80 bpm starts fat burning at 114 bpm. Same age. Different bodies. Different zones.
Now take that runner outdoors. Ten kilometers in 52 minutes flat. Pace: 5:12 min/km. Speed: 11.5 km/h. A solid intermediate result. But the more interesting number comes from a 1981 formula by Peter Riegel: it predicts a full marathon in 3:59:14. Sub-four from a single training run.
And then there is a countdown that has nothing to do with running. 280 days from the first day of a missed period to an estimated due date. A pregnancy calculator that dates from a day you were not yet pregnant - and still manages to predict delivery within a two-week window for 80% of births.
Three calculators. Three different domains. One thing in common: minimal input, maximum output.
190 bpm and five slices of effort
The formula is brutal in its simplicity: HRmax = 220 - age. At 20, your ceiling is 200 bpm. At 50, it drops to 170. Every decade costs you roughly 10 beats.
But the raw number is just the start. The real value comes from dividing it into five training zones. Each zone has a specific purpose, a recommended duration and a distinctive feel.
The Training Heart Rate Calculator offers two methods. The simple one (220 - age) treats everyone identically. The Karvonen method uses your resting heart rate to calculate heart rate reserve and produces personalized zones. The difference is significant:
| Zone | Simple (age 30) | Karvonen (age 30, RHR 55) | Difference |
|---|---|---|---|
| Zone 1 (50-60%) | 95-114 bpm | 123-136 bpm | +28 bpm |
| Zone 2 (60-70%) | 114-133 bpm | 136-150 bpm | +17 bpm |
| Zone 3 (70-80%) | 133-152 bpm | 150-163 bpm | +11 bpm |
| Zone 4 (80-90%) | 152-171 bpm | 163-177 bpm | +6 bpm |
| Zone 5 (90-100%) | 171-190 bpm | 177-190 bpm | +6 bpm |
The lower zones shift the most. A fit person's Zone 2 on Karvonen starts where Zone 3 begins on the simple method. That is not a rounding error - it is the difference between easy jogging and tempo running.
Why does it matter? Because most training plans are built on zones. "Do your long run in Zone 2" means 114-133 bpm on the simple method but 136-150 bpm on Karvonen. Follow the wrong set of zones and you are either sandbagging (too easy) or overtraining (too hard) every single session.
How to measure resting heart rate: first thing in the morning, before getting out of bed, after a normal night of sleep. Lie still for 5 minutes. Count for 60 seconds. Average 3-5 mornings. A smartwatch that tracks overnight HR does this automatically. Retest every 4-8 weeks - as your fitness improves, RHR drops, and your zones shift upward.
5:12 per kilometer and a sub-four marathon
You know your zones. Now you run. Ten kilometers through the park, watch beeping at every split. Final time: 52 minutes, 0 seconds.
The Running Pace Calculator converts that into pace (5:12 min/km), speed (11.5 km/h), and four race predictions. The math behind it is the Riegel formula: T2 = T1 x (D2/D1)^1.06. The exponent 1.06 captures a reliable pattern in human endurance - as distance doubles, pace slows by a predictable percentage.
From that one 10K:
- 5K: 24:56
- 10K: 52:00 (your input - sanity check)
- Half marathon (21.1 km): 1:54:45
- Marathon (42.2 km): 3:59:14
Sub-four marathon. From a training run.
The catch: the formula assumes you train proportionally for the target distance. A 10K specialist who never runs more than 12 km will hit the wall at kilometer 30 of a marathon regardless of what Riegel says. The prediction is a ceiling - achievable with proper long-run training, not a guarantee.
| 5K time | Pace | 10K | Half marathon | Marathon |
|---|---|---|---|---|
| 20:00 | 4:00 | 41:40 | 1:32:13 | 3:12:59 |
| 25:00 | 5:00 | 52:05 | 1:55:17 | 4:01:14 |
| 30:00 | 6:00 | 1:02:30 | 2:18:20 | 4:49:29 |
| 35:00 | 7:00 | 1:12:55 | 2:41:24 | 5:37:44 |
One practical tip: use your race time, not training time. Training runs are 10-20% slower than race effort. No adrenaline, no taper, no crowd pushing you through kilometer 38. If you only have training data, enter a recent hard tempo run.
And for long training runs? Most coaches recommend 60-90 seconds slower than marathon race pace. If Riegel predicts 5:40 min/km for your marathon, your long runs should hover around 6:30-7:00 min/km. Stay in Zone 2 from the heart rate calculator. Running long runs too fast is the single most common training error.
280 days from a date you were not pregnant
The third calculator operates on a completely different timescale. No heart rates. No split times. Just a date and a formula from the early 1800s.
The Pregnancy Calculator takes the first day of your last menstrual period (LMP), adds 280 days, and produces an estimated due date. This is the Naegele rule - named after Franz Naegele, a German obstetrician who published it in 1830.
LMP March 15, standard 28-day cycle:
- Due date: December 20, 2026
- Current week: 15 weeks 2 days
- Trimester: 2nd (weeks 14-27)
- Estimated conception: March 29
- Days since conception: 93
- Days remaining: 173
The counterintuitive part: pregnancy is dated from a day you were not yet pregnant. Weeks 1 and 2 of "pregnancy" are before conception. Ovulation happens around day 14 of a 28-day cycle, so when the doctor says "15 weeks pregnant," the embryo is actually about 13 weeks old.
Cycle length matters more than most people realize. If your cycle runs 35 days instead of 28, ovulation likely occurred around day 21 - a full week later. Without correction, the due date would be 7 days too early. The calculator adjusts automatically.
| Trimester | Weeks | What happens |
|---|---|---|
| 1st | 1-13 | Heartbeat at week 6, organs forming, highest risk period, NT scan at 12 |
| 2nd | 14-27 | First movements (week 18-20), anatomy scan, viability threshold at 24 |
| 3rd | 28-40 | Rapid weight gain, lungs maturing, full-term at 37, due date at 40 |
Only 5% of babies arrive on the exact due date. But 80% land within two weeks. First babies tend to arrive slightly late. An early ultrasound (weeks 8-12) refines the estimate to +/- 5 days.
The common thread
These three calculators share a design philosophy: take 1-3 inputs that anyone can provide (age, a time, a date) and produce results that would otherwise require a coach, a sports physiologist or a doctor's appointment. The formulas behind them - Karvonen (1957), Riegel (1981), Naegele (1830) - have survived decades of validation because they model real biological patterns with surprising accuracy.
The heart rate calculator feeds directly into the pace calculator: your Zone 2 range tells you how fast to run your easy days. Your pace tells you what marathon time is realistic. And if the marathon is nine months away, well - there is a calculator for that countdown too.
Tools discussed in this article
- Training Heart Rate Calculator - five personalized training zones from your age and resting heart rate (Simple or Karvonen method)
- Running Pace Calculator - pace, speed and race predictions for 5K, 10K, half marathon and marathon from any run (Riegel formula)
- Pregnancy Calculator - due date, current week, trimester and countdown from your last menstrual period (Naegele rule)
More Health & Fitness tools
- BMI Calculator - Body Mass Index with WHO classification
- BMR & TDEE Calculator - basal metabolic rate and total daily energy expenditure
- Ideal Weight Calculator - target weight from three clinical formulas
- WHR Calculator - waist-to-hip ratio and cardiovascular risk
- Body Fat Calculator - body fat percentage via U.S. Navy method
- One Rep Max Calculator - 1RM estimation from submaximal lifts
- Calories Burned Calculator - MET-based calorie burn for 23 activities
- Water Intake Calculator - daily hydration target by weight and activity
- Protein Intake Calculator - ISSN-based daily protein needs with per-meal breakdown
Check for a specific age
- Heart rate zones 30 years - training zones
- Heart rate zones 40 years - safe limits
- Resting heart rate 65 bpm - fitness level