Which of the three predictions should I trust?

Use the blended estimate as your best single number. If you'd rather pick one, Cameron is usually closest for the 10K-to-marathon stretch (it was fit on exactly that range), VDOT is best when you've actually trained the long stuff (it assumes the mileage is there), and Riegel is the most optimistic — it ignores fatigue beyond 1.06^x and overshoots if your endurance is weak.

Why does my watch's prediction disagree?

Garmin / Coros race predictors fold in your recent weekly load and VO2max trend. This calculator only sees the one race you typed in. If you ran a fast 5K but only train 30 km/week, the watch will (correctly) predict slower than Riegel. The blended estimate here lands closer to Garmin if your training matches the distance you're predicting to.

How accurate is "5K to full marathon"?

Roughly ±5% if you have the appropriate weekly mileage for a marathon (50+ km/week sustained for 12+ weeks). Without that mileage, all three formulas overestimate by 5–15 minutes because they assume your aerobic base matches your speed. The longer your input race, the tighter the prediction — half marathon to full is typically ±2%.

What is VDOT and why is it the coaching gold standard?

VDOT (Jack Daniels, 2014, Daniels' Running Formula 3rd ed.) is a single number that summarises your race fitness — effectively a VO2max-equivalent back-solved from a race result. Two runners with the same VDOT race the same times across all distances and respond to the same training paces. Coaches use it because once you have one VDOT, every training pace (Easy / Marathon / Threshold / Interval / Repetition) drops out of a single table — no guessing.

Are the training paces shown here from a real Daniels table?

Yes. We use the Daniels intensity coefficients — Easy ≈ 70% VDOT, Marathon ≈ 84%, Threshold ≈ 88%, Interval ≈ 98%, Repetition ≈ 105% — and the standard ACSM oxygen-cost model to convert each back to pace. The numbers match the printed VDOT tables in Daniels' Running Formula within a second or two per kilometre.

My target is sub-3:00 marathon. What weekly mileage does the tool suggest?

The mileage baselines shown are pulled from Pfitzinger / Daniels published plans: roughly 50–60 km/week for sub-4:00, 65–80 km/week for sub-3:30, 80–105 km/week for sub-3:00, and 105–130+ km/week for sub-2:45. These are sustained training weeks in the build phase, not peak weeks alone — and they assume you've held the volume for 8–12 weeks before race day.

Why does the taper plan cut volume so much?

Bosch, Mujika and others have shown that 2–3 week tapers with 40–60% volume reduction (intensity preserved) produce 2–3% performance gains in marathoners. The 3-week pattern shown — 80% / 65% / 50% of peak volume — is the most common evidence-supported template. Cutting more than 60% loses fitness; cutting less leaves residual fatigue.

Is anything sent to a server?

No. Your race time, distance, weekly mileage and goals stay in your browser. We don't log, store or transmit any of it. You can use the tool offline at the start line of a race.

Project a sub-3 marathon from a recent half-marathon PR

You ran 1:24:30 at a local half last weekend. Pick "Half" + that time. The tool returns Riegel ≈ 2:56:30, Cameron ≈ 2:58:10, VDOT ≈ 2:59:20 — blended 2:57:55 (±2:30). VDOT 54.5 falls out the side. Read off the threshold pace (≈ 4:00/km) and the marathon pace (≈ 4:11/km) — those are your tempo session and long run race-pace blocks for the next 12 weeks. The sub-3 mileage baseline (80–105 km/week) tells you whether the half PR is actually sustainable to the full distance.

Decide whether to chase a sub-4 goal from a 5K time-trial

5K time trial reads 23:50. The predictor pegs full marathon around 3:55–4:05 across the three formulas. That puts sub-4 on the table — but only if you're already running 50+ km/week. Open the mileage baseline panel: sub-4 = 50–60 km/week build, peak 65 km. If you're currently at 30 km/week, the 5K speed says you can do it, but the volume says you can't yet. Plan a 12-week ramp at 10%/week before committing to the goal.

Calibrate your easy-run pace so it actually stays easy

Recent 10K race time 47:40 → VDOT 47.5. The Daniels easy pace locks at 5:50–6:25/km. Most amateurs run their easy days at 5:15–5:30/km because it "feels easy" — but at that VDOT it's actually marathon pace, not easy. Tape the 5:50/km ceiling to your watch and the next 4 weeks of easy runs will rebuild aerobic base without grinding into the threshold zone. The payoff shows up in the next race, not the next workout.

Plan race-week taper after 16 weeks at 100 km/week

Marathon is 21 days out and you've been holding 100 km/week. The 3-week taper plan reads 80% / 65% / 50% of peak — so 80 km, 65 km, 50 km. The tool also locks marathon pace from your VDOT (≈ 4:30/km if VDOT is 47) so each taper week's tune-up runs sit at exactly that pace. Three weeks later you line up with the legs fresh but the pace memory intact.

Compare with a friend training at a different distance focus

You ran 39:20 at 10K; your friend ran 1:25:10 half. Plug both. The tool shows your projected marathon is faster on Cameron and Riegel — but your friend's VDOT projection is closer because the half is a tighter prior for the marathon than the 10K is. Useful for deciding who paces whom on the long run; longer-input projections beat shorter-input ones when both runners' weekly volumes are similar.

Marathon Time Predictor — Riegel + Cameron + Jack Daniels VDOT

Marathon time predictor — input 5k/10k/half time, get full marathon prediction via 3 formulas (Riegel/Cameron/VDOT), with pace zones and weekly mileage advice.

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Race distance

Finish time (HH:MM:SS)

Blended marathon prediction

3:37:55

95% range: 3:34:39 — 3:41:12

Per-formula breakdown

Riegel 1981

3:36:13

Classic, slightly fast

Cameron 1998

3:40:29

Best 10K–marathon fit

Jack Daniels VDOT

3:36:13

Coaching gold standard

VDOT 43.0

Daniels training paces (per km)

Your VDOT: 43.0 · A single number summarising race fitness — read off training paces below.

Race	Pace	Notes
Easy (E)	5:46	Conversational base mileage. 80% of weekly minutes.
Marathon (M)	4:59	Race-pace blocks within long runs.
Threshold (T)	4:48	Sustained "comfortably hard". 20-minute tempo.
Interval (I)	4:24	3-5 min reps at ~5K pace. Builds VO2max.
Repetition (R)	4:10	200-400 m fast reps. Builds speed & economy.

Weekly mileage baseline

Pfitzinger / Daniels published plans. Sustain 8–12 weeks pre-race, not just peak.

Goal	Range (km/wk)	Peak
sub-4:00(you)	50–60	65
sub-3:30	65–80	90
sub-3:00	80–105	115
sub-2:45	105–130	145

3-week race-week taper

40–60% volume cut over 3 weeks, intensity preserved (Mujika 2010).

Week	Volume	Notes
3 weeks out	80%≈ 52 km	Last big block. One long run at 80% of peak. Keep one quality session.
2 weeks out	65%≈ 42 km	Volume down hard, intensity intact. Short marathon-pace tune-up.
Race week	50%≈ 33 km	4-5 short easy runs, 1-2 with a few strides. Sleep + carbs.

Real-runner reference table

Profile	5K	Marathon
Beginner(first marathon)	30:00	4:45:00
Amateur(50 km/wk)	24:00	3:55:00
Serious amateur(70 km/wk)	20:30	3:20:00
Sub-3 hopeful(90 km/wk)	18:00	2:58:00
Elite amateur / pro(160+ km/wk)	14:30	2:20:00

Sources: Riegel PS (1981) "Athletic Records and Human Endurance"; Cameron CR (1998); Daniels J (2014) "Daniels Running Formula" 3rd ed.

What this tool does

Free online marathon time predictor. Enter one race you've actually run — any of 5K, 10K, 15K, 10 mile, half marathon, 25K or 30K — and get a full marathon prediction from three respected formulas side-by-side: Riegel 1981 (T2 = T1·(D2/D1)^1.06, the classic, runs slightly fast), Cameron 1998 (adjusts the exponent by your actual race velocity, more accurate over the 5K → marathon stretch), and Jack Daniels VDOT (the coaching gold standard — back-solves your VO2max-equivalent from the race, then projects). The three predictions are blended into a single best estimate with a 95% confidence band. You also get Daniels' six training paces locked to your VDOT — Easy, Marathon, Threshold, Interval, Repetition and Long run — plus baseline weekly mileage for sub-4:00, sub-3:30, sub-3:00 and sub-2:45 goals, and a 3-week taper plan for race week. Everything runs in your browser; no race data leaves the page.

Tool details

Input: Text + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 28 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Student; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Marathon Time Predictor fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Project a sub-3 marathon from a recent half-marathon PR
You ran 1:24:30 at a local half last weekend. Pick "Half" + that time. The tool returns Riegel ≈ 2:56:30, Cameron ≈ 2:58:10, VDOT ≈ 2:59:20 — blended 2:57:55 (±2:30). VDOT 54.5 falls out the side. Read off the threshold pace (≈ 4:00/km) and the marathon pace (≈ 4:11/km) — those are your tempo session and long run race-pace blocks for the next 12 weeks. The sub-3 mileage baseline (80–105 km/week) tells you whether the half PR is actually sustainable to the full distance.
Decide whether to chase a sub-4 goal from a 5K time-trial
5K time trial reads 23:50. The predictor pegs full marathon around 3:55–4:05 across the three formulas. That puts sub-4 on the table — but only if you're already running 50+ km/week. Open the mileage baseline panel: sub-4 = 50–60 km/week build, peak 65 km. If you're currently at 30 km/week, the 5K speed says you can do it, but the volume says you can't yet. Plan a 12-week ramp at 10%/week before committing to the goal.
Calibrate your easy-run pace so it actually stays easy
Recent 10K race time 47:40 → VDOT 47.5. The Daniels easy pace locks at 5:50–6:25/km. Most amateurs run their easy days at 5:15–5:30/km because it "feels easy" — but at that VDOT it's actually marathon pace, not easy. Tape the 5:50/km ceiling to your watch and the next 4 weeks of easy runs will rebuild aerobic base without grinding into the threshold zone. The payoff shows up in the next race, not the next workout.
Plan race-week taper after 16 weeks at 100 km/week
Marathon is 21 days out and you've been holding 100 km/week. The 3-week taper plan reads 80% / 65% / 50% of peak — so 80 km, 65 km, 50 km. The tool also locks marathon pace from your VDOT (≈ 4:30/km if VDOT is 47) so each taper week's tune-up runs sit at exactly that pace. Three weeks later you line up with the legs fresh but the pace memory intact.
Compare with a friend training at a different distance focus
You ran 39:20 at 10K; your friend ran 1:25:10 half. Plug both. The tool shows your projected marathon is faster on Cameron and Riegel — but your friend's VDOT projection is closer because the half is a tighter prior for the marathon than the 10K is. Useful for deciding who paces whom on the long run; longer-input projections beat shorter-input ones when both runners' weekly volumes are similar.

Common pitfalls

Predicting a marathon from a 5K when your weekly mileage is under 40 km/week. All three formulas assume your aerobic base matches your speed; without the mileage, real-world marathon times come out 10–20 minutes slower than predicted. Use the mileage baseline panel as a reality check before locking in a goal.
Treating Riegel as the truth. Riegel's 1.06 exponent fits an "average trained runner" — if you're new to long distance, the actual exponent for you is closer to 1.08 or 1.10, and Riegel will overshoot. Trust the blended estimate, especially when 5K input is your only data.
Running easy days at marathon pace instead of Daniels easy pace. Easy pace from the VDOT table is 30–60 seconds per km slower than marathon pace for most amateurs. Running easy at marathon pace is the single most common cause of stalled improvement and overuse injury — slow down on the easy days, the speed shows up on race day.

Privacy

Your race distance, finish time, weekly mileage and goal pace are all processed entirely in your browser. We do not send any of this to a server, do not log it, and do not store it after you close the tab. The Daniels VDOT table, Riegel and Cameron formulas all live in the JavaScript bundle — no API calls, no analytics on your inputs. You can use the predictor at the start line of a race in airplane mode and get identical numbers to running it online.

FAQ

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Marathon Time Predictor — Riegel + Cameron + Jack Daniels VDOT

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Marathon Time Predictor fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Project a sub-3 marathon from a recent half-marathon PR

Decide whether to chase a sub-4 goal from a 5K time-trial

Calibrate your easy-run pace so it actually stays easy

Plan race-week taper after 16 weeks at 100 km/week

Compare with a friend training at a different distance focus

Common pitfalls

Privacy

FAQ

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