Regulo Mark 5 means 190 °C, or 375 °F. That one line answers the question my whole copy of a 1974 British cookbook kept asking me, page after page, without ever explaining itself. I bought the book for almost nothing at a Sunday book bazaar in Multan, from a stall where cookbooks sat in a tea chest between car manuals and school atlases. The cover had lost its jacket. The pages smelled of storage and spice tins. And every single recipe spoke a language my Celsius oven dial does not speak: Regulo 3, Regulo 5, a warm oven, a brisk oven, nothing else.
My first attempt went the way you would expect. I fed one recipe into a random conversion site, the site skipped the half marks and rounded in a hurry, and my fruit cake came out undercooked by a full mark. A cake can survive many insults. Raw batter in the middle is not one of them. That evening I printed three conversion tables from three sources and found they disagreed with each other at Gas Mark 3, of all places. So I stopped trusting strangers with this book. I sat down, worked out a table I could defend mark by mark, and wrote the answers into the book itself. This article is that work, shared for anyone holding a similar book.
What The Regulo Numbers Actually Mean
Regulo was not a recipe writer's nickname for anything. It was the name of the thermostat control fitted to British gas cookers in the early twentieth century, and the numbered settings on that control became the Gas Mark scale the whole country cooked by. When a 1974 recipe says Regulo Mark 5, the writer is telling you the exact dial position on the cooker she assumed stood in your kitchen. The number is the instruction. Nothing about it is vague once you know the scale exists.
The scale runs lower and finer than most modern readers expect. Gas Mark 1/4 is 110 °C. Gas Mark 1/2 is 120 °C. Gas Mark 1 is 140 °C, and each whole mark after that adds roughly 10 to 20 °C until Gas Mark 9 reaches 240 °C. Cookbooks of that period used the quarter and half marks without apology, because the dials of the period carried them. A modern converter that starts at Gas Mark 1 has already failed the reader of an old book, which is why the Gas Mark to Celsius converter on this site starts at 1/4 and keeps every fraction the old dials ever had.
The old books were not being difficult. They were writing for a dial that every reader owned, and the dial simply is not the one in your kitchen now.
Three Recipes I Had To Rescue
The fruit cake came first, at Regulo Mark 5. Converted honestly, that is 190 °C, and at 190 °C the second attempt held together, browned at the edges, and cut clean. The cake had never been the problem. My first conversion had quietly handed me a cooler oven than the writer asked for, and a rich fruit cake mix is heavy enough to punish that gap.
The second rescue was a casserole described with the line that made the least sense to me in the whole book, quoted word for word: "Bake in a warm oven, Regulo 3, until set." It turned out to mean 170 °C, held steady, a gentle braising heat that the phrase alone would never have told me, and the casserole has worked every single time since I learned that. The third was a baked custard at Regulo Mark 1/2, 120 °C, where the recipe expected the custard to set over a long, patient hour. At modern default settings most cooks would have scorched it in twenty minutes and blamed the eggs.
Those three recipes taught me the pattern of the whole book. The writer trusted two systems at once: a word for the feel of the oven, and a Regulo number for the setting. Modern readers get the word and panic, or get the number and guess. The book expects you to hold both, and the table in the next section is the whole vocabulary in one place. Where a book gives the word with no number beside it at all, the word has to be tested bake by bake, the job I did for My Aunt's Recipe Cards That Say Moderate Oven.
The marks my 1974 book uses most, in Celsius
Values: standard published conversion table, as listed by BBC Good Food.
The Whole Temperature Vocabulary Of The Book
Old British cookbooks speak in a small, fixed vocabulary. Once I listed the phrases from my copy and matched each one to the marks the writer used beside them, the book stopped being a puzzle. Here is the vocabulary with the settings I now use.
What the book says
Gas Mark
Celsius
Fahrenheit
Very slow oven
1/2
120 °C
250 °F
Slow oven
2
150 °C
300 °F
Warm oven
3
170 °C
325 °F
Moderate oven
4
180 °C
350 °F
Fairly hot oven
6
200 °C
400 °F
Hot oven
7
220 °C
425 °F
Very hot oven
9
240 °C
475 °F
One warning belongs with this table. Writers of that era were consistent inside one book and sloppy across books. A warm oven in my 1974 copy is Regulo 3 because the writer pairs the phrase with the number on the same page. In a book that gives only words, treat the table as a starting point and watch the first bake closely. The fractional marks matter here too, and my guide to Quarter And Half Gas Marks covers the settings that sit between the whole numbers in this table.
The Use Case: A Family Book Lands On Your Shelf
Picture the real situation this article exists for. An aunt, a grandmother, or a charity shop hands you a British cookbook printed before 1980. The recipes are the ones your family actually ate, which is why you want the book at all. The first recipe you open says Regulo 4 and nothing else, your oven says 180, and your phone offers you a converter that starts at Gas Mark 1 and rounds everything. You do what I did at the start: you guess, and the food pays for the guess. The fix takes ten minutes once. Convert the book's ten most used settings, write them inside the front cover in your own hand, and the book becomes yours for life. Every recipe after that is just cooking.
The same era of book usually measures ingredients by weight in ounces or by the cup, depending on which side of the Atlantic printed it. When a recipe from the chest gives you grams and your kitchen works in cups, the same front-cover treatment works there too, and this site's grams to cups converter exists for exactly that shelf of books.
What I Wrote In The Margins
Book collectors would call what I did vandalism. I call it the book finally doing its job. Inside the front cover of my 1974 copy there is now a small handwritten table: every Regulo mark the writer uses, the Celsius beside it, and one line that says warm means 3 in this book. Three recipes in the book carry a pencil tick and a date, the mark of a rescue that worked. The custard page has a coffee ring on it that I refuse to feel bad about.
If you take one habit from this article, take that one. A conversion you look up is a conversion you will look up forty times. A conversion you write into the book becomes part of the book, and the next reader, who might be cooking from it in another fifty years, inherits your work along with the recipes. That is how these books survived this long in the first place. Somebody in 1974 trusted the reader to own a Regulo dial. Somebody now has to trust the reader with a table instead.
Questions People Ask About This Page
Is Regulo The Same Thing As Gas Mark?
Yes. Regulo was the brand name of the thermostat control fitted to British gas cookers, and the numbers on a Regulo dial are the Gas Mark numbers. Regulo Mark 5 and Gas Mark 5 both mean 190 °C.
Why Do Old British Cookbooks Never Print Celsius?
Home ovens in Britain carried Regulo number dials for decades, and Celsius dials only became common later. A book printed in 1974 assumed every reader owned the same numbered dial, so the author had no reason to print a second scale.
My Old Book Just Says A Moderate Oven. What Number Is That?
Moderate usually means Gas Mark 4, which is 180 °C. Fairly hot points to Gas Mark 6 at 200 °C, and a slow oven sits near Gas Mark 2 at 150 °C. When a dish is delicate, start one mark lower and give it more time.
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