Here's an easy way to work our your predicted Ironman time:
Take your half Ironman time, double it and add an hour if your time is under 5 hours. If it's over 5 hours, add 90 minutes.
Now, the technicall bit: If you don't own a powermeter, or ignore your powermeter, or don't know how to use your powermeter, or ignore your heart rate monitor, or believe you can race by feel and all this technobabble is rubbish, add on another 30 minutes.
If you're an alpha male and like to zoom past people on the bike, add another 20 minutes.
If you are convinced that sports nutrition is not necessary and malt loaf will do just fine, add on another 15 minutes.
If you didn't practice changing punctures, add on another 15 minutes.
If you didn't get your bike serviced and at least change your chain, add on another 10 minutes.
If you decided to ride in new shorts, or decided to save 30 seconds in transition by not putting some chamois cream on, don't add any time on, but it sure will feel like you've added 30 minutes.
If you decided to run in new shoes you bought two days earlier at the expo and tried out for a 20 minute run and they felt fine, add on another 15 minutes.
If you bought calf guards at the expo and are trying to put them on in T1, good luck to you making the cut off
I'm only being slightly tongue in cheek here
oxygenaddict.com Triathlon Coaching
I always enjoy this question and really liked Rob;'s comments. He covered everything except of course to factor in the weather. Which you can only do on race day in the UK as its the only time you might have a chance of predicting that.... Cheers Jay