Friel Heart Rate Zone Calculator

Discipline:

Use the discipline that produced the threshold: Cycling or Running.

Threshold HR: {{ effectiveThresholdDisplay }}

Enter 90-230 bpm as a whole number; use your latest repeatable LTHR.

bpm

Test average:

Enter the average bpm from the selected 20-60 minute test, or leave blank.

bpm

Test method:

Use 20-minute, 30-60 minute, tempo, or lab/direct to match the test source.

Apply test override:

On uses test average times factor; Off uses Threshold HR.

Current HR: {{ current_hr || 0 }} bpm

Enter 0-240 bpm; use 0 to hide the current-HR marker.

bpm

Session duration:

Enter total workout minutes, for example 45, 60, or 90.

min

Session focus:

Pick Recovery, Endurance base, Tempo, Threshold, or VO2 / 5K.

Discipline offset:

Enter signed bpm, for example -5 for a lower bike LTHR or 4 for a higher run LTHR.

bpm

Zone	Percent of LTHR	BPM range	Purpose	Copy
{{ z.id }}	{{ z.percent_display }}	{{ z.bpm_display }}	{{ z.cue }}

Zone	Minutes	Percent	Intent	Copy
No session plan yet Set a session duration above 0 minutes to build time-in-zone rows.
{{ row.id }}	{{ row.minutes_display }}	{{ row.percent_display }}	{{ row.intent }}

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Heart rate zones give endurance work a repeatable intensity language. Easy recovery, aerobic endurance, tempo pressure, threshold work, and very hard intervals all ask for different effort ranges, and a single average heart rate cannot describe those jobs well.

Friel zones anchor those ranges to lactate threshold heart rate, or LTHR. LTHR is not maximum heart rate and it is not an age formula. It is the heart rate associated with a hard, sustained effort near the point where lactate production and clearance are close enough that trained athletes can often continue for roughly 20 to 60 minutes.

LTHR: The sport-specific threshold heart rate used as the percentage anchor.
Zone boundary: A percentage of LTHR converted into a rounded bpm edge.
Current HR: A live or remembered heart-rate value compared with the calculated zone table.
Sport specificity: Running and cycling thresholds should be tested and applied separately.

Running and cycling can produce different threshold heart rates for the same athlete. Running usually involves more body mass and impact, while cycling is shaped by posture, pedal force, cooling, and local muscle fatigue. Copying a bike threshold into a run plan can make easy running too hard. Copying a run threshold into a bike workout can make threshold intervals miss the intended load.

Zone math is only as reliable as the threshold behind it. A recent field test, race, or lab assessment is more useful than a stale number from another training phase. Heat, dehydration, altitude, caffeine, stress, sleep, fatigue, and sensor fit can all move heart rate away from the same pace or power, so one odd workout does not automatically invalidate a zone table.

A heart-rate range is a target, not a command. Breathing, perceived effort, terrain, workout purpose, and recovery status still matter. Stop exercising and seek qualified advice if unusual symptoms appear, if heart rate behaves unexpectedly, or if a clinician has given exercise limits.

How to Use This Tool:

Start by choosing the discipline that produced the threshold value. Cycling and Running use different lower-zone boundaries, so this choice changes the table even when the threshold bpm stays the same.

Choose Cycling or Running to match the sport used for the threshold test, race, or lab value.
Enter Threshold HR when you already have a recent LTHR value. The main input is intended for whole-number values from 90 to 230 bpm.
A manual threshold above 230 bpm triggers a warning. Recheck the value, the unit, and whether you entered maximum heart rate instead of LTHR.
Use Test average, Test method, and Apply test override when you want the calculator to estimate LTHR from a field-test average instead of the manual threshold.
The override uses the selected test factor before any signed discipline offset is added.
Open Advanced when you need planning fields. Current HR marks the matching zone, Session duration creates time-in-zone rows, Session focus chooses the distribution, and Discipline offset adds or subtracts bpm after the threshold source is resolved.
Read Friel Zone Bands first. It gives the zone ID, percent of LTHR, bpm range, and training purpose for each band.
Use Session Plan when session minutes are above zero. Use Friel Zone Range Chart and Friel Zone Map to inspect the bands visually, especially when checking a current heart rate against the threshold.
Copy or download the zone table, session table, chart image, DOCX report, or JSON after the warning area is clear and the summary shows the expected threshold or current-HR zone.

Interpreting Results:

The summary reports either the resolved threshold or the zone that contains the optional current heart rate. The threshold source line matters because a manual LTHR, a 20-minute test average, a longer time trial, a steady-tempo estimate, and a lab value do not all enter the calculation the same way.

Threshold is the effective LTHR after any field-test factor and signed offset have been applied.
Friel Zone Bands is the main prescription table. Rounded bpm boundaries can create one-beat gaps or overlaps, so treat the ranges as practical targets rather than lab cut points.
Current HR is a lookup marker. It identifies the matching zone but does not change the calculated bands.
Session Plan allocates total minutes across the selected focus preset. It is a planning scaffold, not a full workout prescription with warm-up detail, interval recovery structure, or terrain adjustments.
JSON mirrors the current result with discipline, threshold, threshold source, zone ranges, and session allocation for reuse outside the page.

Use the result as a repeatable planning aid. If the same sport, test protocol, sensor, and conditions are used over time, shifts in threshold can show training adaptation. If any of those inputs changed, compare results cautiously before rewriting a training plan.

Technical Details:

Friel-style zones are threshold-relative. Each boundary is a percentage of effective LTHR, then the boundary is rounded to a whole beat per minute for display. That rounding makes the table usable on a watch or head unit, but it also means the exact edge between two zones should not be treated as a physiological switch.

The threshold is resolved before any zones are built. A manual LTHR can be used directly, or a field-test average can be multiplied by the selected method factor. A signed offset is then added for known sport-to-sport differences, and the effective threshold is capped at 260 bpm.

Formula Core:

The primary mechanism is threshold resolution followed by percentage boundaries. In the formulas below, source is either the manual threshold or the field-test average after its method factor.

{LTHR}_{effective} = \min (260, {LTHR}_{source} + offset)

{BPM}_{lower} = round ({LTHR}_{effective} \times p_{lower})

{BPM}_{upper} = round ({LTHR}_{effective} \times p_{upper})

Displayed Friel zone percentage boundaries by discipline
Zone	Cycling percent of LTHR	Running percent of LTHR	Training emphasis
Z1	65-81%	65-85%	Recovery and easy aerobic work
Z2	82-89%	86-89%	Endurance volume
Z3	90-93%	90-94%	Tempo and sustained aerobic pressure
Z4	94-99%	95-99%	Sub-threshold work
Z5a	100-102%	100-102%	Super-threshold intervals
Z5b	103-106%	103-106%	VO2-focused intervals
Z5c	107-112%	107-112%	Anaerobic efforts

Threshold source factors used before zone calculation
Threshold source	Cycling factor	Running factor	Use case
20-minute field test	0.95	0.98	Reduces a shorter hard average before zones are built
30-60 minute TT or race	1.00	1.00	Uses a longer threshold-style average directly
Long steady tempo	0.93	0.97	Converts a steadier effort into an LTHR estimate
Lab or direct LTHR	1.00	1.00	Uses a direct threshold value

For example, a cycling 20-minute test average of 170 bpm resolves to 161.5 bpm before any offset. Rounded from that effective threshold, Z4 is 152-160 bpm and Z5a is 162-165 bpm. The same 170 bpm entered as a lab or direct LTHR would produce higher ranges because the 0.95 field-test adjustment would not be applied.

Session focus distributions used for time-in-zone planning
Focus	Distribution
Recovery	70% Z1, 30% Z2
Endurance base	20% Z1, 55% Z2, 15% Z3, 10% Z4
Tempo	15% Z1, 25% Z2, 40% Z3, 15% Z4, 5% Z5a
Threshold	10% Z1, 20% Z2, 25% Z3, 25% Z4, 15% Z5a, 5% Z5b
VO2 / 5K	10% Z1, 15% Z2, 15% Z3, 20% Z4, 20% Z5a, 20% Z5b

The session table multiplies total minutes by the chosen focus distribution and rounds each row to one decimal minute when needed. A 60-minute Endurance base plan assigns 12 minutes to Z1, 33 minutes to Z2, 9 minutes to Z3, and 6 minutes to Z4.

Advanced Tips:

Use 20-minute field test only when the entered average comes from a hard 20-minute effort. A 30 to 60 minute time trial or lab value should use a 1.00 factor.
Keep Discipline offset at zero unless repeated testing shows a consistent sport difference. Guessing an offset can shift every band in the wrong direction.
Set Current HR to zero when you only need a zone table. Enter a value when you want the summary, chart, and zone map to mark a specific heart rate.
Choose Session focus after setting the total minutes. The session rows distribute time across zones but do not add warm-up structure, recoveries, terrain, or coach-specific interval rules.
Use the chart image or DOCX export for training notes, and use JSON when you need the threshold, source, zones, and session rows in a structured format.

Limitations, Privacy, and Accuracy Notes:

The calculation runs in the browser from the values on the page. It does not need an account or a server lookup to produce the zone table. Exported files and copied data contain the values currently shown in the result.

Heart rate responds slowly during short surges, so very brief intervals may be better guided by pace, power, perceived effort, or the coach's instructions.
Optical wrist sensors can lag, lock onto cadence, or misread some sessions. A well-fitted chest strap is often more stable for threshold testing.
Retest after a training block, injury break, equipment change, major climate shift, or a long gap from structured training before comparing old and new zones.
These ranges are training aids. They are not medical clearance, diagnostic thresholds, or a substitute for individual coaching or clinical advice.

Worked Examples:

Cycling field test with endurance planning

A rider selects Cycling, enters a 170 bpm Test average, keeps 20-minute field test selected, and leaves Apply test override on. The threshold source becomes 170 x 0.95, or 161.5 bpm before rounding. With a 60 minute Session duration and Endurance base focus, the session plan puts most of the time in Z2 with smaller Z1, Z3, and Z4 blocks.

Running threshold with a current-HR lookup

A runner selects Running, enters a manual Threshold HR of 165 bpm, turns off the test override, and sets Current HR to 158 bpm. The running table rounds Z4 to 157-163 bpm, so the summary marks the current heart rate as Z4.

Offset for a known discipline difference

An athlete with a tested cycling LTHR that is usually 5 bpm lower than running can enter the resolved threshold and use a -5 bpm Discipline offset only when that adjustment reflects their own testing history. Guessing an offset is weaker than retesting in the correct sport.

FAQ:

Should I choose Cycling or Running?

Choose the sport that produced the threshold. The lower-zone percentages differ, and the current-HR lookup uses the same sport-specific table.

Why does the 20-minute field test use a factor?

A short hard average can sit above the heart rate a person could hold for a longer threshold effort. The calculator multiplies a 20-minute average by 0.95 for cycling or 0.98 for running before building zones.

Can I use a lab threshold value?

Yes. Select Lab or direct LTHR with a valid test average, or turn off Apply test override and enter the lab value as Threshold HR.

Why do the displayed bpm ranges skip a beat near some boundaries?

The calculator rounds each lower and upper boundary independently. Use the rounded ranges as practical training targets, and avoid treating a one-beat edge as a meaningful physiological line.

Why is the Session Plan empty?

The Session Plan tab needs a Session duration above zero. Leave duration at zero when you only need the zone bands.

Glossary:

LTHR: Lactate threshold heart rate, the threshold anchor used to calculate percentage-based heart rate zones.
Field test: A hard sport-specific effort used to estimate threshold outside a lab setting.
Effective threshold: The threshold after any field-test factor and signed bpm offset have been applied.
VO2: High-intensity aerobic capacity work, represented in this calculator mainly by Z5a and Z5b time.
Offset: A signed bpm adjustment added after the threshold source is resolved.

References:

Joe Friel's Blog: A Quick Guide to Setting Zones, TrainingBible.
Joe Friel's Quick Guide to Setting Zones, TrainingPeaks.
Zones Calculator Overview, TrainingPeaks Help Center.
Lactate Profile, UC Davis Sports Medicine.
Heart Rate Monitors: How to Choose and Use, REI Expert Advice.