Ok,so at the moment i have 5lb leader and tippet on all 3 of my lines but next week i will have the chance on something huge lets say 20lbs.
Now i dont want the thing to get off so what leader do i need? 20lb?

of weight may not be appropriate for a fish that size (an educated guess from my fishing here in the PNW). But to your question. For Steelhead at/under 10#'s I'm usually running 8, to as low as 6 pound tippet with a loose drag.

Add higher water flows, and larger fish I'll jump to 10-12 pounds. REALLY big fish, as we're now seeing here on the Rogue (Spring run King Salmon) then the 10wt 2-handers come out and I'm running at 15#'s.

'But it's a 30# fish Fred, 15 pound leader sounds darned light.'

Not really. Remember (here anyway) we set the drag on our reels at/just above half the breaking strength of the tippet material. Your equipment takes up the 'slack' above that of the leader strength. As a test of this, rig your rod/line with even a 12# tippet, strip off 40' of line off the rod tip and see if you can break same. Darned near impossible unless you're the size of a PNW Lumber Jack weighing in at 18-25 "Stone." (A 'Stone' is 14 pounds if memory serves.)

fae

19.6 lb Seaguar fluoro tippet would be what I'd choose...

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Save the softmouth trout

Use Maxima, stuff is tough as nails. Up at that strength, I'll usually make my own 'leaders' out of Maxima main line rather that a 'leader' product.

Just a thought.

fae

To get the best answer to this you would be best to supply some more info. When you say the fish you will have a shot at are 20lb what are they? Stocked rainbows, salmon, trevally etc. Is where you're going to be fishing free from snags or do you need to keep a tight rein?

What species are you fishing for? and what type of water are you fishing?Is there a lot of weed growth/underwater snags?What is your rods action? a fast actioned rod will require a heavier tippet as it will not cushion the surges of a large fish and will result in break offs with lighter tippets. A softer action will allow you to drop down a couple of lbs. This is also handy if you are going to be fishing dries or smaller nymphs as you will need a lighter tippet to allow a more natural movement.
On stillwater for rainbows - A fast action rod and lure you could probably go up to 8lb mono
A soft action rod and dries/nymphs i would stick to 5/6lb fluoro and use the rod and drag to play the fish.
It is a real balancing act between presentation and fish welfare, you are better to er on the side of caution you probably would'nt want to lose a big fish.

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rod is a 6weight stocked brown and rainbows on a stillwater,rod action is med i think,its a hardy demon.The fish are upto 20lb i probably wont be lucky enough to hook one,ill be using a lure i think.

I'd say you're there or there abouts with 5lb, even for larger fish on a small still-water. I use 6lb for all my still water and reservoir fishing (fluro for sub surface and mono for surface work). Had fish to 18lb without too many issues. For sure you'll always run the risk of a smash take (it's usually the initial take and strike that puts the greatest amount of strain on a leader) but you tend to find it isn't the doubles that behave like that anyway. You also have to balance out the positives (less chance of a break) against the negatives (less likelihood of tempting spooky fish, usually doubles which have been targeted a great deal and have become a little wiser) when choosing leader strength/diameter.

19.6 was a minimum - often not leader shy in the wild - 25.5 lb is the next step up.

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Save the softmouth trout

is it right that you cant tie flouro tippet to co poly leader?