How It Works

This tool projects performance from one distance to another using a distance-to-time scaling model that runners often use for quick estimates. The idea is simple: as distance increases, you cannot usually hold the same relative speed, so the projected time grows slightly faster than distance. The model captures that slowdown with a single exponent.

You enter a recent race distance and your finish time, select a target distance (marathon by default), and choose an exponent. The exponent acts like an endurance dial. A lower exponent assumes you slow less as distance increases (strong endurance and solid long-run volume). A higher exponent assumes you slow more (less endurance-specific training, tough course conditions, or an input race that was not truly maximal).

The tool also calculates average pace per kilometer and per mile because pace is what you actually run. A finish time is useful for a goal, but pace helps you execute workouts and a race-day plan. If you turn on splits, you also get checkpoint times in 5 km blocks or 1 mile blocks, which are easy to copy into a pacing band or notes app.

Step-by-step workflow

• 1. Choose your input distance: Pick a race you ran recently and hard enough to represent current fitness. A recent 10K or half marathon is often a good anchor.
• 2. Enter the time: Use HH:MM:SS for longer events (example: 1:52:00) or MM:SS for shorter races (example: 24:15). For MM:SS, minutes can be greater than 59.
• 3. Select the target: Keep Marathon, switch to Half Marathon or 10K, or choose a custom distance for a 30K or an endurance benchmark.
• 4. Set the exponent: Start with 1.06 as a balanced default, then adjust based on your endurance profile and how comparable your input race is to the target.
• 5. Generate the estimate: Review finish time, average pace, a practical range, and optional splits. Copy the summary or download it as a text file.

Key Features

Predict marathon finish time from real performance

Instead of guessing a goal based on wishful thinking, you start from an actual race result. The predictor scales your performance to marathon distance and returns a clear finish-time estimate. This helps you avoid the common trap of setting a marathon goal that is too aggressive for your current training cycle.

If you use a half marathon or 10K that you raced all-out, the estimate typically lands in a reasonable zone for goal-setting. If your input was a relaxed effort or took place in challenging conditions, you can adjust the exponent to produce a more conservative plan.

Average pace per kilometer and per mile

Marathon pacing is easier when you have one primary pace target and a small allowable range. The tool computes average pace per kilometer and per mile so you can match the units you use in training. If you run with kilometer splits on your watch, focus on the per-kilometer pace. If your route and race signage are in miles, use the per-mile pace.

Pace outputs are also valuable for training design. Marathon-pace workouts, steady-state runs, and long-run segments can be planned around a pace target rather than a finish-time guess. That keeps training specific and measurable.

Practical optimistic-to-conservative range

No single number can account for weather, terrain, taper quality, fueling, and day-of execution. For that reason, the result panel includes a small range around the main estimate by nudging the exponent slightly in both directions. The faster number can represent an excellent day, strong endurance, and good conditions. The slower number can represent a tougher course, heat, headwinds, pacing mistakes, or under-fueling.

Use the range as a planning tool: pick an A-goal near the faster end only if training evidence supports it, a B-goal around the main estimate, and a C-goal near the conservative end so you have a confident fallback strategy when conditions are unfavorable.

Optional splits for pacing checkpoints

Even pacing is one of the simplest ways to maximize your marathon potential. If you enable splits, the predictor creates checkpoint times at regular intervals (every 5 km or every mile). These checkpoints are based on average pace across the target distance. They help you keep early effort controlled, avoid a too-fast first 10K, and stay on track when the race gets noisy and crowded.

Splits are also useful for support crews and friends. Sharing predictable checkpoint times makes it easier for them to know when to expect you, and it gives you a simple structure for mentally breaking the race into manageable sections.

Copy-ready and download-friendly output

The summary output is formatted as plain text so you can copy it into a training log, send it to a coach, or save it alongside your marathon plan. If you want an offline reference, you can download the summary as a text file. That makes it easy to keep the estimate on your phone, print it, or attach it to a spreadsheet-based training planner.

Use Cases

• Set a realistic marathon goal time: Use a recent race to choose A/B/C goal times and avoid starting a cycle with an unrealistic target.
• Translate race results into training paces: Convert the predicted finish time into marathon-pace guidance for tempo blocks, long-run segments, and steady runs.
• Build a pacing plan with checkpoints: Generate 5 km or mile splits so you can keep early pace controlled and reduce the risk of late-race fade.
• Compare tune-up races during a cycle: Re-run the predictor after a 10K or half marathon to see whether marathon projections are trending in the right direction.
• Plan fueling and hydration timing: Estimate time on course so you can schedule gels, fluids, and caffeine more precisely, especially if you aim for intake every 25–35 minutes.
• Estimate custom distances: Use the custom target for a 30K race, a long training benchmark, or an endurance event where you still want a road-style pace plan.
• Adjust expectations for course difficulty: If your target marathon is hilly or you expect warm weather, choose a slightly more conservative exponent and plan pace accordingly.

In practice, the best way to use a projection is as a decision aid, not a rule. Many runners can “hit” a projection when training volume is strong and the input race is close in duration to the marathon. Others will need a conservative plan because their endurance training is still developing. The predictor helps you see the difference and choose a strategy you can execute confidently.

If you work with a coach, the tool also provides a shared baseline. Coaches often combine projections with training data such as long-run pace, heart rate drift, and how you recover from marathon-pace workouts. With a consistent baseline, it is easier to discuss whether your goal should be ambitious, moderate, or cautious.

Optimization Tips

Pick an input race that matches your current fitness

Projections work best when the input reflects what you can do right now. If your race was several months ago, or it happened before a major training block, your fitness may have changed significantly. Whenever possible, use the most recent race or time trial that you completed at a strong, steady effort.

Also consider course and weather. A hilly race or a hot day can slow your time even when fitness is good. In those cases, either use a different input race or choose a slightly lower exponent only if you believe the conditions suppressed performance in a way that will not apply to your target marathon.

Use the exponent like an endurance adjustment

The default exponent of 1.06 is a practical starting point, but you can tune it. If you consistently run strong negative splits and you have months of steady mileage, you may find that a slightly lower exponent produces a projection that better matches your performance at longer distances. If you tend to fade late, have inconsistent long runs, or are new to marathon training, a slightly higher exponent is often safer.

When in doubt, choose conservative. A conservative goal time supports a smoother race where you can finish strong. Starting too fast, even by a small margin, can lead to large time losses in the last third of the marathon.

Validate the estimate with long-run evidence

A prediction is most useful when it aligns with training. As your cycle progresses, compare the predicted marathon pace to how marathon-pace segments feel during long runs. If you can hold the pace comfortably while fueled, and you recover well afterward, the prediction is likely realistic. If the pace feels like a maximum effort in training, it may be too ambitious for race day.

Use long-run structure to test sustainability: for example, a steady long run with a final segment near marathon pace, or a long run with alternating blocks slightly slower and slightly faster than marathon pace. These sessions provide strong feedback about what you can execute on tired legs.

FAQ