Fishing fly sizes can seem confusing when you first start fly fishing. Unlike many measurements, **the larger the hook size number, the smaller the fly generally becomes**.
A size 18 fly is therefore much smaller than a size 10, while a size 6 is larger again.
Once you understand this basic principle, fly sizing becomes considerably easier.
## What Does the Fly Size Number Mean?
When an angler describes a fly as a **size 14**, they are normally referring to the size of the hook on which the fly has been tied.
For example:
- Size 20 - very small.
- Size 18 - small.
- Size 16 - small to medium.
- Size 14 - medium.
- Size 12 - medium to large.
- Size 10 - large.
- Size 8 - larger again.
These descriptions are only a rough guide. A size 12 dry fly and a size 12 streamer can look completely different because the hooks themselves may have very different shapes and proportions.
## Are Hook Sizes Standard?
Not completely.
A size 14 from one hook manufacturer may not be exactly the same dimensions as a size 14 from another.
Hook style also makes a substantial difference.
A size 12 short-shank hook may produce quite a compact fly, while a size 12 long-shank streamer hook could produce a fly several times longer.
This means the number should be treated as a **useful indication of size rather than a precise measurement of the finished fly**.
## Why Does Fly Size Matter?
Size is one of the most important characteristics of a fishing fly.
When imitating natural food, getting the approximate size right can sometimes be more important than matching every detail of colour or shape.
If trout are feeding on small upwinged flies, presenting a very large imitation may look completely wrong regardless of how accurately it is tied.
Likewise, when fishing an attractor or lure, changing to a larger or smaller fly can significantly alter how fish respond.
## Fly Size and Presentation
The size of the fly also affects how it casts and fishes.
A small dry fly requires relatively little casting energy and can be presented on fine tippet.
A large Mayfly pattern has considerably more air resistance and requires more energy to turn over properly.
Large streamers, predator flies and saltwater flies can create an even greater casting payload and normally require heavier tackle, stronger leaders and more powerful fly lines.
This is why the size of the flies you intend to cast should always be considered when choosing a fly fishing outfit.
## Choosing Fly Size for River Trout and Grayling
For UK river trout and grayling fishing, anglers commonly use flies from around **size 10 down to size 20 or smaller**, depending on the insect being represented and the fishing conditions.
A large Mayfly imitation might be tied around size 10, while small midges and tiny upwinged flies may require size 18, 20 or smaller hooks.
Rather than automatically choosing a particular size, look at the natural insects and other food available and choose a fly that provides a reasonable match.
## Choosing Fly Size for Stillwater Trout
Stillwater trout flies cover a particularly broad range of sizes.
Small buzzers and nymphs may be tied on relatively small hooks, while larger lures, streamers and attractor patterns can be considerably bigger.
Fly size may be chosen according to natural food, fishing depth, water clarity, weather conditions or simply the response of the fish on the day.
## What Happens Beyond Size 1?
Hook sizing becomes slightly different once you reach the larger sizes.
After size 1, larger hooks are normally described using **aught sizes**, written as:
- 1/0
- 2/0
- 3/0
- 4/0
In this part of the sizing system, the rule reverses - **the larger the number before /0, the larger the hook**.
A 4/0 is therefore considerably larger than a 1/0.
These larger hooks are commonly encountered in predator and saltwater fly fishing.
## Fly Size Is More Than the Hook Number
Two flies tied on exactly the same hook size can have completely different profiles.
Consider:
- The length of the materials.
- The amount of dressing.
- The hook length.
- The width of the fly.
- The amount of hackle.
- The size of wings or tails.
- The overall silhouette.
A sparsely dressed size 12 spider can therefore appear much smaller than a heavily dressed size 12 lure.
This is why experienced anglers tend to think about the **overall size and profile of the fly**, rather than relying entirely on the hook number.
## Key Takeaways
- Fishing fly size normally refers to the hook size.
- Generally, the larger the hook number, the smaller the fly.
- A size 18 is smaller than a size 12.
- Hook sizes are not perfectly standard between manufacturers or hook styles.
- Long-shank and short-shank hooks can produce very different sized flies despite carrying the same hook number.
- Once hook sizes reach 1/0 and above, increasing numbers indicate larger hooks.
- The overall profile of the finished fly is often more important than the hook number alone.
- When imitating natural food, matching the approximate size can be one of the most important parts of choosing the right fly.
In the next article, we'll look at **barbless vs barbed fishing flies** and explain the practical differences between the two.
