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Half Marathon & Marathon Pace Planner

Plan your half marathon & full marathon pace with custom split strategies, mile or km splits, target finish times, fueling alerts, and Riegel race predictions.

100% Client-Side Privacy
Lbl quick presets
Lbl race distance
Lbl target finish time03:59:00
Lbl hours
Lbl minutes
Lbl seconds
Lbl pacing strategy
Lbl weather condition14°C

Lbl ideal weather

Kpi projected finish
03:59:00
42.2 Unit km / 26.2 Unit mi
Kpi target pace
05:40Unit min km
10.59 Unit kmh
Kpi total carbs
239Unit grams
~10 Unit packets (956 Unit kcal)
Kpi halfway split
01:59:30
Badge halfway
Col segment:
Col segmentCol distanceCol paceCol split timeCol elapsed timeCol speedCol fueling
Lbl km segment 11.0 Unit km05:40 Unit min km05:4000:05:4010.6 Unit kmh-
Lbl km segment 22.0 Unit km05:40 Unit min km05:4000:11:2010.6 Unit kmh-
Lbl km segment 33.0 Unit km05:40 Unit min km05:4000:17:0010.6 Unit kmh-
Lbl km segment 44.0 Unit km05:40 Unit min km05:4000:22:3910.6 Unit kmh-
Lbl km segment 55.0 Unit km05:40 Unit min km05:4000:28:1910.6 Unit kmhDesc gel water
Lbl km segment 66.0 Unit km05:40 Unit min km05:4000:33:5910.6 Unit kmh-
Lbl km segment 77.0 Unit km05:40 Unit min km05:4000:39:3910.6 Unit kmh-
Lbl km segment 88.0 Unit km05:40 Unit min km05:4000:45:1910.6 Unit kmh-
Lbl km segment 99.0 Unit km05:40 Unit min km05:4000:50:5910.6 Unit kmh-
Lbl km segment 1010.0 Unit km05:40 Unit min km05:4000:56:3910.6 Unit kmhDesc gel water
Lbl km segment 1111.0 Unit km05:40 Unit min km05:4001:02:1810.6 Unit kmh-
Lbl km segment 1212.0 Unit km05:40 Unit min km05:4001:07:5810.6 Unit kmh-
Lbl km segment 1313.0 Unit km05:40 Unit min km05:4001:13:3810.6 Unit kmh-
Lbl km segment 1414.0 Unit km05:40 Unit min km05:4001:19:1810.6 Unit kmh-
Lbl km segment 1515.0 Unit km05:40 Unit min km05:4001:24:5810.6 Unit kmhDesc gel water
Lbl km segment 1616.0 Unit km05:40 Unit min km05:4001:30:3810.6 Unit kmh-
Lbl km segment 1717.0 Unit km05:40 Unit min km05:4001:36:1710.6 Unit kmh-
Lbl km segment 1818.0 Unit km05:40 Unit min km05:4001:41:5710.6 Unit kmh-
Lbl km segment 1919.0 Unit km05:40 Unit min km05:4001:47:3710.6 Unit kmh-
Lbl km segment 2020.0 Unit km05:40 Unit min km05:4001:53:1710.6 Unit kmhDesc gel water
Lbl km segment 2121.0 Unit km05:40 Unit min km05:4001:58:5710.6 Unit kmh-
Lbl km segment 22Badge halfway22.0 Unit km05:40 Unit min km05:4002:04:3710.6 Unit kmh-
Lbl km segment 2323.0 Unit km05:40 Unit min km05:4002:10:1710.6 Unit kmh-
Lbl km segment 2424.0 Unit km05:40 Unit min km05:4002:15:5610.6 Unit kmh-
Lbl km segment 2525.0 Unit km05:40 Unit min km05:4002:21:3610.6 Unit kmhDesc gel water
Lbl km segment 2626.0 Unit km05:40 Unit min km05:4002:27:1610.6 Unit kmh-
Lbl km segment 2727.0 Unit km05:40 Unit min km05:4002:32:5610.6 Unit kmh-
Lbl km segment 2828.0 Unit km05:40 Unit min km05:4002:38:3610.6 Unit kmh-
Lbl km segment 2929.0 Unit km05:40 Unit min km05:4002:44:1610.6 Unit kmh-
Lbl km segment 3030.0 Unit km05:40 Unit min km05:4002:49:5610.6 Unit kmhDesc gel water
Lbl km segment 3131.0 Unit km05:40 Unit min km05:4002:55:3510.6 Unit kmh-
Lbl km segment 3232.0 Unit km05:40 Unit min km05:4003:01:1510.6 Unit kmh-
Lbl km segment 3333.0 Unit km05:40 Unit min km05:4003:06:5510.6 Unit kmh-
Lbl km segment 3434.0 Unit km05:40 Unit min km05:4003:12:3510.6 Unit kmh-
Lbl km segment 3535.0 Unit km05:40 Unit min km05:4003:18:1510.6 Unit kmhDesc gel water
Lbl km segment 3636.0 Unit km05:40 Unit min km05:4003:23:5510.6 Unit kmh-
Lbl km segment 3737.0 Unit km05:40 Unit min km05:4003:29:3410.6 Unit kmh-
Lbl km segment 3838.0 Unit km05:40 Unit min km05:4003:35:1410.6 Unit kmh-
Lbl km segment 3939.0 Unit km05:40 Unit min km05:4003:40:5410.6 Unit kmh-
Lbl km segment 4040.0 Unit km05:40 Unit min km05:4003:46:3410.6 Unit kmhDesc gel water
Lbl km segment 4141.0 Unit km05:40 Unit min km05:4003:52:1410.6 Unit kmh-
Lbl km segment 4242.0 Unit km05:40 Unit min km05:4003:57:5410.6 Unit kmh-
Lbl finish segment (42.2 Unit km)42.2 Unit km05:40 Unit min km01:0603:59:0010.6 Unit kmh-
Kpi target pace05:40 Unit min km
Lbl heat adjusted05:40 Unit min km
Kpi gels needed10 Unit packets
Kpi hydration target~2.4 Unit liters
Kpi halfway split01:59:30
Lbl total race burn~2743 Unit kcal
Lbl athletic notice titleLbl athletic disclaimer

Client-Side Processing

High-performance calculation executed entirely in browser memory.

Zero Server Upload Privacy

All payloads remain strictly local with zero external server communication.

Production Ready Export

Clean, optimized output formats ready for immediate deployment.

ToolsAA•100% Client-Side Private Web Utilities
Technical Reference & Architecture Guide

Half Marathon & Marathon Pace Planner Guide

Plan your half marathon & full marathon pace with custom split strategies, mile or km splits, target finish times, fueling alerts, and Riegel race predictions.

Half Marathon & Marathon Pace Planner Guide — Client-Side In-Memory Architecture & Data Flow
Half Marathon & Marathon Pace Planner Guide — Client-Side In-Memory Architecture & Data Flow

Key Parameters & Controls

Target Race Distance (Official Course Length)Race Distance Category

Calibrated to official World Athletics sanctioned track and road dimensions: Half Marathon (21.0975 km / 13.1094 mi) or Full Marathon (42.1950 km / 26.2188 mi), strictly accounting for the 0.1% Short Course Prevention Factor required by AIMS course measurement certifiers.

Default: Full Marathon (42.195 km / 26.219 mi)
Target Finish Time & Average VelocityNet Elapsed Chip Time (HH:MM:SS)

The exact net gun-to-mat duration representing the total aerobic work budget. Average velocity defines the core required split pace before application of topographic variance or split strategy offsets.

Default: 03:59:00 (Sub-4:00 Marathon)
Split Strategy Profile (Velocity Distribution)Mathematical Pacing Variance Distribution

Determines the split trajectory across the first and second half. Even Pacing models 0% split difference; Negative Split models a +1% to +6% conservative start with second-half acceleration; 3-Phase Build models conservative settlement (0-10km), steady aerobic cruise (10-32km), and threshold kick (32km+).

Default: Negative Split (-2.0% Acceleration)
Riegel Fatigue Exponent (Aerobic Base Index)Endurance Conversion Power Exponent (b)

The exponent b in Riegel's formula T2 = T1 * (D2 / D1)^b. Standard aerobic baseline is 1.06. High-mileage runners with 70+ miles/week and high mitochondrial density achieve 1.04-1.05; recreational athletes with weekly volume under 40 miles experience progressive muscle damage and require 1.07-1.09.

Default: 1.06 (Standard Aerobic Base)
Exogenous Carbohydrate Ingestion RateCarbohydrate Fueling Rate (Grams / Hour)

Target oxidation rate for exogenous fuel. The human liver and skeletal muscle store approximately 400-500g of glycogen (~2,000 kcal), which is fully exhausted around 30-32 km at marathon pace without supplemental carbohydrates. Benchmark recommendations are 30-60 g/hr (single-transporter glucose) or up to 90 g/hr (2:1 maltodextrin to fructose dual-transporter ratio).

Default: 60 g/hr (2-3 Energy Gels per Hour)
Thermal Strain & Dew Point Adjustment FactorEnvironmental Temperature & Humidity Penalty

Calculates metabolic slow-down according to ACSM wet-bulb benchmarks. Running economy and cardiac output degrade by 1.0% to 3.5% for every 5°C (9°F) increase above the optimal marathon performance range of 8°C to 12°C (46°F to 54°F).

Default: 14°C (57°F) Nominal Temperature

Pro Tips & Best Practices

The GPS Distance Drift Fallacy (Why Your Watch Reads 42.6 km)

Never pace your race using your watch's instantaneous GPS distance. Official courses are certified along the shortest mathematical path (the tangents of every curve). In a crowded marathon, weaving through runners and taking wide corners adds 1.0% to 1.8% extra distance (typically 42.5 km to 42.8 km total). If you target 5:41/km on your watch, you will cross the official 42.195 km timing mat at 4:02:30. Always calibrate your pacing band 2 to 4 seconds faster per kilometer (5:38/km) to guarantee an official chip-time sub-4:00.

The 15-Kilometer Adrenaline Trap & Glycogen Preservation

Running just 5 to 10 seconds per kilometer faster than target pace during the first 10-15 km feels effortless due to race-day taper and glycogen fullness. However, running above aerobic threshold shifts substrate utilization heavily from free fatty acid oxidation to intramuscular glycogen. Every excess carbohydrate molecule burned early cannot be replenished at high intensities, accelerating the onset of 'The Wall' at kilometer 32 by up to 45 minutes.

Dual-Transporter Gut Saturation Rule (60g vs 90g Carbs)

The intestinal SGLT1 transporter saturates at approximately 60 grams of glucose/maltodextrin per hour. Consuming 80-90 grams of pure glucose causes osmotic fluid draw into the gut, inducing severe cramping, nausea, and dehydration. To safely absorb 75 to 90 grams per hour for sub-3 marathon efforts, your fuel must utilize a 2:1 or 1:0.8 glucose-to-fructose ratio leveraging the secondary GLUT5 fructose transport pathway.

Quadriceps Eccentric Damage Prevention on Downhill Starts

Courses with significant downhill first halves (such as Boston or St. George) entice runners to 'bank time'. Excessive downhill impact forces cause micro-tears in quadriceps muscle fibers through eccentric loading. By kilometer 28, cellular calcium leakage and membrane disruption cause severe muscular failure, regardless of cardiovascular fitness. Never run downhills more than 3% faster than your average target race pace.

Common Gotchas & Troubleshooting

⚠️Hitting 'The Wall' at Kilometer 32 (Severe 60-90s/km Pace Collapse)

This is almost universally physiological glycogen depletion combined with cellular muscle damage, not a lack of willpower. Immediately consume a fast-acting liquid carbohydrate gel with sodium and water, drop cadence by 5% while keeping strides compact, and focus on bilateral arm drive. For your next race cycle, increase peak long runs to 32-35 km, practice race-pace nutrition in training, and increase carbohydrate intake to >=60g/hr starting at minute 30.

⚠️Acute Gastrointestinal Cramping or Nausea from Energy Gels

Taking concentrated energy gels without adequate plain water creates a hypertonic solution in the stomach, stalling gastric emptying. Always wash down each gel packet with 150-200 ml of plain water (never full-strength sports drinks, which compounds hypertonicity). If nausea strikes, shift to small sips of electrolyte fluid every 10 minutes and rinse your mouth with carbohydrate solution to maintain motor cortex central drive via oral carbohydrate receptors without overloading the gut.

⚠️Erratic GPS Watch Pace Bouncing Under Urban High-Rises or Bridges

Urban canyons (such as the Chicago, London, or Tokyo Marathons) deflect satellite signals, causing instantaneous pace to bounce wildly between 3:30/km and 7:00/km. Turn off your watch's auto-lap feature and switch the display screen to 'Elapsed Time' and 'Lap Pace'. Manually press the lap button at every official physical kilometer or mile marker sign along the course and cross-check against your printed pace band.

⚠️Extreme Early Heart Rate Drift in Hot or Humid Conditions (>18°C / 64°F)

Cardiac drift occurs when peripheral blood flow is diverted to the skin for evaporative cooling, reducing stroke volume and forcing heart rate up by 10-15 bpm for the identical pace. Do not chase your cool-weather pace target. Immediately apply a 2% to 4% pace deceleration (8 to 15 seconds slower per km), douse water over your head and wrists at every aid station, and consume 500-750 mg of sodium per liter of fluid to sustain plasma volume.

Frequently Asked Questions

How do I calculate my exact marathon pace and kilometer splits?▼

Divide your total target finish time in seconds by the official race distance (42.195 km for a marathon, 21.0975 km for a half marathon). For example, a 3:45:00 marathon equals 13,500 seconds / 42.195 km = 319.94 seconds per km, or 5:20 min/km (8:35 min/mi). To calculate splits, apply a pacing model: an even pace maintains 5:20/km throughout, whereas a 2% negative split begins at 5:23/km for the first 21.1 km and progressively accelerates to 5:17/km for the second half.

What half marathon time do I need to run a sub-4 hour full marathon?▼

Under Riegel's endurance formula (T2 = T1 * [42.195 / 21.0975]^1.06), a runner with balanced aerobic stamina needs a half marathon personal best of approximately 1:53:30 (5:23 min/km / 8:40 min/mi) to break 4 hours in the full marathon. Runners with lower weekly training volume (<40 miles per week) experience steeper late-race cardiac drift and typically require a 1:50:00 to 1:51:00 half marathon time to achieve a sub-4:00 marathon.

Why is a negative split faster than an even or positive split in a marathon?▼

A negative split is physiologically superior because running the first half 1% to 3% conservatively prevents early lactate accumulation, minimizes core body temperature elevation, and preserves intramuscular glycogen by maximizing fat oxidation during the first 90 minutes. In contrast, 'banking time' with a fast first half floods muscle fibers with hydrogen ions, accelerates eccentric muscular degradation, and invariably causes a catastrophic 20% to 40% pace collapse in the final 10 kilometers.

How much does hot weather and humidity slow down marathon race pace?▼

Exercise physiology research from the American College of Sports Medicine (ACSM) shows that marathon pace deteriorates by approximately 1.5% to 2.0% for every 5°C (9°F) increase above 12°C (54°F). At 20°C (68°F) with high humidity, a 3:30 marathoner (5:00/km pace) should proactively adjust their target pace by 12 to 18 seconds slower per kilometer (5:12 to 5:18/km) to avoid heat exhaustion, core overheating, and severe cardiac drift.

When should I take my first energy gel during a marathon?▼

Take your first energy gel between 45 and 60 minutes into the race (around kilometer 8 to 10), well before you feel fatigued or depleted. Ingesting 20 to 30 grams of carbohydrates every 30 to 45 minutes sustains circulating blood glucose and spares liver and skeletal muscle glycogen. Waiting until kilometer 25 or 30 is the primary cause of gastrointestinal shutdown and late-race bonking, as digestion slows dramatically once systemic dehydration and muscular strain set in.

How do elevation gain and hills affect marathon pacing strategy?▼

Disclaimer: This tool is provided strictly for educational and informational purposes only and does not constitute formal financial advice or a binding loan commitment. Please consult a qualified financial advisor before making decisions. Run uphills by perceived aerobic effort and heart rate rather than clinging to a fixed GPS pace, allowing velocity to decrease by 15 to 30 seconds per kilometer depending on slope. Every 1% increase in uphill gradient raises oxygen cost by approximately 3% to 4%, burning through finite glycogen reserves. On downhills, lean forward from the ankles and maintain quick, light turnover without overstriding, keeping pace within 3% to 5% of target to protect knee joints and quadriceps muscle fibers for the finish.

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