You don't need bread flour to make decent bread, plain flour doesn't generally have as much gluten content
but it has enough to make bread that's perfectly fine.
You won't get the same texture as with bread flour, it won't be as elastic a result.
Gluten is a protein, strong flour has more protein in it than plain flour, but there isn't always that much difference between the two.
It depends on the brand.
In my cupboard my plain flour has 10g of protein and my strong 12.5g, that's not that much more.
In the past I've had plain flour with a lot less, like 6g, so your plain flour may have more gluten than you think.
If your plain is a little weak you can use your strong flour to bulk up the gluten content, a 30/70 mix will give a small boost and a 50/50 mix more so.
But, remember if plain is all you have you can still make bread.
You may want to reduce your hydration a bit with lower protein, and give it a longer proving time to develop the gluten you have.
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I agree with @GdD's answer, and it should be the main answer, but I wanted to throw in another bit of advice.
If you find that your all purpose flour does not give you the results that you want, look for vital wheat gluten.
It's less likely that people will be hoarding that.
Adding a bit of wheat gluten should make AP flour act much more like bread flour.
This is the trick bakers use for modern whole wheat and rye breads, as they contain much less gluten per weight.
In fact, wheat gluten is used frequently by low-carb bakers to create bread-like products without adding the starch of flour.
You can find recipes online to work from, but the recommended amount seems to be 1 tablespoon for every 2 cups (about 9 oz / 256 grams) of flour.
If you want to use a recipe you have and are particular, you can calculate how much protein is in the amount of flour in your recipe and add 1/4 as much ( of the protein, not the flour! )
vital wheat gluten by weight.
You may want to decrease the amount of flour by the same amount -- 1 tablespoon or whatever you calculated -- to maintain your hydration level. https://www.cooksillustrated.com/how_tos/5867-vital-wheat-gluten
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"Bread flour" (or actually the wheat cultivar used for it) is a North American invention that has spread also to Great Britain.
In most other countries, home bakers are not even aware of the existence of bread flour, and bake bread with the equivalent of all-purpose flour.
You can use all purpose flour almost all your bread recipes without any change.
Only the highest hydrations (85% upward) will give you structural trouble.
If all you have had until now was bread made with bread flour, the new texture will be unusual, but if you approach it with openmindedness, you will soon get used to it.
If you either have too strong an averse reaction to bread that is similar, but
not exactly the way you like it, or want to try new things, I suggest that you simply go for European or Near East bread recipes and make those.
There is a huge variety of breads to be found there, and most of them are traditionally made without bread flour.
Just make sure to not use recipes that American bakers have modified to use bread flour.
You might have to (or enjoy to!)
branching out to breads made with flours from other plants too, not just wheat.
Beside the variety, it seems that the less common grains don't disappear from the supermarket shelf as quickly in a hamster run.
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By "plain" I assume you mean AP (all-purpose) flour.
There are many great bread recipes that use AP flour.
There will be less gluten development with AP than with bread flour, so you may want to knead a bit longer depending on your recipe, but it should come out fine in any case.
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Making bread is fun and easy, if you have kids, it is a good life skill to show them how to make some.
Yes you can use regular flour
, I use regular unbleached flour.
If you have yeast, then you can make simple no knead bread ; most recipes I've seen use 3 cups of flour, yeast, salt and water.
If you do not have yeast, then you can start to make a sourdough starter .
(disclaimer, I don't have an link with that site, they were the top answer from google).
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Yes, plain flour (or mixed) will work.
The resulting loaf will probably not store well, it will become very hard.
No problem when eaten fresh.
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So... Since the lock-down everybody turned baker (can't understand why, since there is plenty of bread in the shops) and I can't find bread flour anymore.  Is there a way to use plain flour instead?  I have 1.5kg of strong flour left, can I mix it with plain flour and get decent results?
How to make bread with plain flour?
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Bread flour has more gluten and will produce a more elastic texture, but it is perfectly possible to make bread with normal flour and in many countries this is the norm. American bread recipes call for bread flour but recipes from many other countries use all purpose flour. American recipes make need to be tweaked in order to make them successfully with all purpose flour.
All purpose flour is what most people use to make bread outside North America. Choose a recipe that is not American for a good result. The texture is not the same as bread made with bread flour so a few changes may be needed to the recipe and it may take time to get used to it.