The official symbol for the gram in the International System of Units (SI) is simply “g,” not “gm.” While “gm” appears routinely in recipes, informal notes, and older texts, it has never been recognized by the SI as a valid symbol for the gram. The two are often treated as interchangeable in casual settings, but they are not formally equivalent, and the difference matters more than you might expect once precision, digital systems, or medical dosing enter the picture.
What the SI Actually Specifies
The International System of Units assigns every unit a fixed symbol. For the gram, that symbol is a lowercase “g” with no trailing letters. These symbols are not abbreviations in the way “Dr.” abbreviates “Doctor.” They are standardized, language-independent shorthand agreed upon internationally. That means they do not change across languages, do not take a period at the end, and do not get pluralized. One gram is 1 g. Five hundred grams is 500 g. Never 500 gs, 500 gms, or 500 gr.
The reason “gm” was never adopted is partly structural. SI symbols for units are kept as short as possible, typically one or two characters, and every additional character risks collision with the system’s prefix notation. In SI, a lowercase “m” placed before a unit symbol means “milli,” or one-thousandth. So “mg” already means milligram, “mm” means millimeter, and “ms” means millisecond. Adding a stray “m” to a unit symbol is not just non-standard; it invites misreading. “gm” could theoretically be parsed as “gram times meter” in dimensional analysis, or simply confuse someone scanning a document quickly.
Where “gm” Comes From
If “gm” is wrong by SI standards, why does it show up everywhere? The answer is that informal abbreviations for units predate the modern SI by a long time. Before the SI was formalized in 1960, scientists and engineers worked with a patchwork of measurement systems, including the centimetre-gram-second (CGS) system that dominated physics for over a century. The transition from CGS and related frameworks to the modern SI was gradual, stretching across decades, and older abbreviation habits persisted long after the formal rules changed.1Metrologia. A Brief History of the Electrical Units to 1964 During that transition, “gm” and “gms” circulated as common shorthand in English-speaking countries, where it felt natural to abbreviate “gram” the same way you might abbreviate “gallon” to “gal.”
Cookbook authors, pharmacists, and even some laboratory manuals used “gm” well into the late twentieth century. Once an abbreviation gets embedded in a field’s everyday practice, it takes generations to fully fade, especially in informal writing where nobody consults an SI reference sheet before jotting down a measurement.
Why the Difference Matters in Practice
In your kitchen, writing “250 gm flour” instead of “250 g flour” will not ruin your bread. Anyone reading it understands what you mean. But outside casual use, the distinction between official and unofficial notation has real consequences.
The most immediate risk involves SI prefixes. Every SI prefix attaches directly to the unit symbol. Milligram is “mg.” Microgram is “µg.” Kilogram is “kg.” If someone is already in the habit of writing “gm” for gram, then what does “mgm” mean? In some older American medical literature, “mgm” was actually used to mean milligram. That kind of ambiguity is exactly what the SI was designed to eliminate. Research on prospective science teachers has found that unit conversion errors are common, with particular difficulty around conversions between grams and milligrams or grams and micrograms, suggesting that non-standard notation only deepens existing confusion.2Science Education International. Dealing with Metric Unit Conversion: An Examination of Prospective Science Teachers’ Knowledge of and Difficulties with Conversion
In computing and data systems, non-standard symbols create a different problem. Software that parses unit symbols expects the SI-standard versions. A database column full of “gm” entries may not be recognized as grams by an automated system expecting “g.” Studies of SI symbols in computing contexts have highlighted that even official SI notation contains some inherent ambiguities, such as the lowercase “m” representing both “meter” and “milli” depending on context.3ScienceDirect (Computer Standards & Interfaces). Quantities, units and computing Adding unofficial variants like “gm” to the mix only compounds the parsing challenge.
Medical and Pharmaceutical Stakes
The field where sloppy unit notation causes the most harm is medicine. Drug dosing depends on getting the unit right, and the margin between a therapeutic dose and a dangerous one can be slim. The Institute for Safe Medication Practices (ISMP) in the United States has long maintained a list of error-prone abbreviations that should never appear on medication orders. “gm” is among the abbreviations flagged as potentially confusing, precisely because it can be misread or mistaken for other shorthand in handwritten prescriptions.
Hospitals and pharmacies have largely moved to electronic prescribing, which enforces standardized unit symbols. But handwritten notes, patient instructions, and compounding records still circulate in many settings. When a prescriber writes “gm” and a pharmacist reads it quickly, the risk of misinterpretation is small for grams specifically but part of a broader pattern where non-standard abbreviations create openings for error. The push toward “g” and “mg” as the only acceptable symbols in clinical documentation is not pedantry. It reflects hard-won lessons from medication errors that stemmed from ambiguous shorthand.
Microgram notation is an especially sensitive area. The correct SI symbol is “µg,” but because the Greek letter mu (µ) is hard to produce on many keyboards and easy to misread in handwriting, some writers have substituted “mcg.” While “mcg” is not SI-standard either, it has been explicitly accepted in medical contexts as a safer alternative to the error-prone “µg” written by hand. The point is that in clinical settings, clarity trumps tradition, and non-standard symbols get scrutinized much more carefully than in a recipe blog.
Cooking and Everyday Use
Recipes are the single biggest reservoir of “gm” in daily life. If you browse cookbooks published in India, the UK, or Australia, you will find “gm” and “gms” used interchangeably with “g.” Some food bloggers use “gm” because it feels more like a complete word to their audience, especially readers who might not immediately recognize “g” as meaning grams rather than, say, a typo.
There is nothing dangerous about this in a cooking context. The quantities involved are large enough that no one is going to confuse 200 gm of sugar with 200 mg. The only practical annoyance arises when you try to use a digital kitchen scale that expects you to select “g” as the unit, or when a recipe-scaling app does not recognize “gm” as a valid input. These are minor friction points, but they illustrate the broader principle that standardized symbols exist to make communication seamless across tools and contexts.
If you are writing a recipe for personal use, “gm” will not cause any confusion. If you are writing for publication or for an international audience, “g” is the cleaner choice because it is universally recognized and consistent with every style guide that addresses the topic.
Other Abbreviations That Trip People Up
The “gm” versus “g” question sits in a family of similar confusions. Here are some of the most common ones:
- ml vs. mL: Both are technically acceptable for milliliter. The SI permits either a lowercase “l” or an uppercase “L” for liter because the lowercase version looks too much like the numeral “1” in many typefaces. Most style guides now prefer “mL” for clarity.
- cc vs. mL: In older medical writing, “cc” (cubic centimeter) was used interchangeably with “mL” for liquid volumes. They represent the same quantity, but “cc” has been flagged as error-prone and is being phased out of clinical use.
- kg vs. kgs: SI symbols never take an “s” for plural. It is “50 kg,” not “50 kgs.” The same rule applies to every unit: meters stay “m,” liters stay “L,” seconds stay “s,” regardless of quantity.
- sec vs. s: The SI symbol for second is “s,” not “sec.” Like “gm” for gram, “sec” is an informal abbreviation that persists in casual writing but is not SI-recognized.
The unifying theme is that SI symbols are compact, fixed, and do not behave like ordinary English abbreviations. They do not take periods, they do not pluralize, and they do not vary. Once you internalize that principle, you stop wondering whether it should be “gm” or “g” and just use “g.”
The Kilogram Quirk
One genuinely odd thing about the gram is that it is not itself the base unit in the SI. The kilogram is. This makes the gram unusual: it is the only common SI unit whose base form already carries a prefix. “Kilo” means thousand, so a kilogram is literally a thousand grams, and the kilogram, not the gram, is the fundamental unit from which the others are derived.
This quirk creates a naming inconvenience. When you need to express one-thousandth of a gram, you say “milligram,” attaching the prefix to “gram.” But you would never say “millikilogram” for the same quantity, because prefixes in the SI are not stacked. The result is that all prefix-based gram quantities (milligram, microgram, nanogram) are built from “gram” as if it were the base unit, even though formally it is not. This historical oddity traces back to the original metric system of the 1790s, where the gram was defined first and the kilogram was simply a convenient larger unit. When the SI was formalized, practical considerations kept the kilogram as the base unit because the physical reference standard, a platinum-iridium cylinder kept in France until 2019, was a kilogram mass.
None of this changes the fact that the symbol for gram is “g.” But it helps explain why unit notation in the metric system can feel slightly less intuitive than you would expect from a system designed to be logical.
What Style Guides Say
If you are writing for any context that follows a formal style guide, the answer is unambiguous. The American Medical Association (AMA) style manual specifies “g” for gram. The American Chemical Society (ACS) style guide specifies “g.” The Chicago Manual of Style recommends SI symbols for scientific quantities and would therefore use “g.” The Associated Press Stylebook uses “grams” spelled out in most contexts but defers to SI symbols in technical material. No major English-language style guide endorses “gm.”
Academic journals uniformly require SI notation in manuscripts. If you submit a chemistry paper with “gm” in the methods section, a copy editor will change it to “g” before publication. The same applies to physics, biology, engineering, and nutrition journals. In academic and professional writing, the question is settled and has been for decades.
International Variation
Usage of “gm” is not evenly distributed around the world. It appears most frequently in Indian English, where decades of textbooks and commercial packaging used “gm” or “gms” as the default abbreviation for grams. If you buy a packet of spices in India, there is a good chance the label reads “100 gm” rather than “100 g.” This is not because Indian standards differ from the SI. India’s Bureau of Indian Standards follows SI conventions and specifies “g.” But informal commercial practice has its own momentum, and “gm” remains deeply embedded in everyday Indian English writing.
In British, Australian, and American English, “gm” also appears but less pervasively. It tends to show up in older texts, handwritten notes, and informal online writing rather than in printed packaging or official documents. European countries that use the metric system natively tend to stick with “g” in all contexts, partly because their educational systems have taught SI notation for longer and partly because the abbreviation “gm” does not arise as naturally in languages other than English.
If you encounter “gm” on a product label or in a document, there is no ambiguity about what it means. Everyone understands it refers to grams. The issue is not comprehension but standardization: in any context where precision, professionalism, or international communication matters, “g” is the right choice, and “gm” marks the writing as informal or outdated.
Digital Assistants and Search Engines
One modern wrinkle worth noting is how digital tools handle “gm.” If you type “500 gm to oz” into a search engine, most search engines will correctly interpret “gm” as grams and return the conversion. Google’s unit converter, Wolfram Alpha, and similar tools have been trained to recognize “gm” as an informal variant. So for quick lookups, “gm” works fine.
Where it gets messier is in structured data, coding, and APIs. If you are building a recipe database, a nutrition-tracking app, or any system that needs to parse unit strings programmatically, “gm” is a wild card. Some libraries recognize it; others do not. The international standard for representing units in data interchange is the Unified Code for Units of Measure (UCUM), which uses “g” for gram and does not define “gm.” Any system built to UCUM standards will reject “gm” as an unrecognized unit. For anyone working with data, the safest practice is to normalize all gram entries to “g” on input.