How Many Grams Is a Zip? The Exact Figure, and Where It Stops Being True

A zip is one avoirdupois ounce of cannabis: 28.349523125 grams, exactly. The international pound has been defined as 0.453 592 37 kilograms since 1 July 1959, NIST prints that value as an exact figure in Handbook 44, Appendix C, and a pound holds 16 ounces, so the ounce falls out of the division with no rounding anywhere in it. Half a zip is 14.17 g and a quarter zip is 7.09 g; four zips make a quarter pound at 113.40 g, and sixteen make a pound at 453.59 g. Calling a zip 28 g gives up 0.35 g, about 1.2 %. That shortcut holds until it meets a scale, a label, or a statute — NIST Handbook 133 permits a single package declared at 28 g to weigh 25.2 g, and California writes its possession limit as 28.5 grams instead of as an ounce.

The number has a paper trail

Six national standards laboratories settled the ounce in one document. On 1 July 1959, the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed to define the yard as 0.9144 metre and the avoirdupois pound as 0.453 592 37 kilogram. The US notice ran in the Federal Register that day, volume 24, number 128, page 5348, under the title "Refinement of Values for the Yard and the Pound." NIST Handbook 44 still carries the figure in Appendix B and underlines it in Appendix C to mark it exact.

Divide by 16 and you get 28.349523125 g. The decimal terminates, which is why the number is long and why it is not an approximation of anything. Below the ounce the definitions keep going: an ounce is 437.5 grains, and a grain is 64.79891 milligrams.

Here is the part I find quietly funny. NIST's own guide to the SI, Special Publication 811, lists the conversion in Appendix B.9 as 2.834 952 E-02 kilograms per ounce — seven digits, and not in the boldface the document reserves for exact values. The rounding starts inside the agency that holds the exact figure.

One trap worth naming before the arithmetic goes anywhere else: this is the avoirdupois ounce. The troy ounce used for gold and silver is 31.1034768 g, heavier by 2.75 g, and the US fluid ounce is a volume of 29.5735295625 millilitres, which weighs whatever the liquid in it weighs. Cannabis statutes and cannabis labels mean the avoirdupois ounce every time.

Every fraction of a zip, in grams

| Amount | Ounces | Exact grams | Usually quoted as | Difference | |---|---|---|---|---| | Eighth | 1/8 | 3.543690390625 | 3.5 g | 0.04 g | | Quarter zip | 1/4 | 7.08738078125 | 7 g | 0.09 g | | Half zip | 1/2 | 14.1747615625 | 14 g | 0.17 g | | Zip | 1 | 28.349523125 | 28 g | 0.35 g | | Quarter pound | 4 | 113.3980925 | 112 g | 1.40 g | | Pound | 16 | 453.59237 | 448 g | 5.59 g |

A quarter pound is four zips, and a pound is sixteen. The right-hand column multiplies while nobody watches it: invisible at an eighth, worth noticing at a pound.

A label that says 28 g does not promise 28 g

This is the fact most people are missing, and it is written down in public.

NIST Handbook 133, Checking the Net Contents of Packaged Goods, is the document weights-and-measures inspectors use across the United States, adopted by reference in state law — California incorporates the 2023 edition through Title 4 of its regulations. Appendix A, Table 2-5 sets the Maximum Allowable Variation for packages labelled by weight. For any labelled quantity under 36 g, which covers every eighth, quarter, half and zip on a shelf, the MAV is 10 % of the labelled quantity.

Ten per cent of 28 g is 2.8 g. A single package declared at 28 g can lawfully hold 25.2 g.

That number does not stand alone. Handbook 133 applies a second test at the same time, the Average Requirement: the average net contents of packages in a lot must at least equal the declared quantity. Individual packages are allowed to dip; the batch is not. The handbook is also specific about the equipment, telling inspectors to use a scale whose division is no larger than one sixth of the MAV, which for a 28 g package works out to 0.47 g.

Canada draws the line somewhere else. Health Canada's Tolerance Limits for the Net Weight and Volume Declared on Cannabis Product Labelling, in force since 17 October 2019 under section 133 of the Cannabis Regulations, allows 10 % on declared amounts up to 2 g and 5 % above that. A Canadian package labelled 28 g may weigh 26.6 g. Same word, same plant, two regulators, and a 1.4 g gap between the floors they set.

Moisture accounts for a real share of the movement. Handbook 133, section 1.2.6, permits deviations caused by moisture loss or gain from ordinary exposure in good distribution practice, and notes that permeable packaging makes desiccation likelier. Dried flower trades water with the air around it. The rain that started here this afternoon, and settled the question of whether I was going anywhere, is the same variable that Handbook 133 is describing: a jar in a humid room and the same jar in a dry one will not read alike on the same scale, and neither reading is dishonest.

If you want to check a package against its own label, the procedure is short.

1. Read the declared net weight in grams off the label, not the count of units or the THC figure.
2. Weigh the contents alone, with all packaging off the pan, on a scale that reads to 0.01 g.
3. Compare the difference against the tolerance that applies where you are — 10 % of the label below 36 g under Handbook 133, or 5 % above 2 g under the Health Canada limits.

Possession limits are written in grams, or in ounces, and never in zips

No statute I have read uses the word. Legislators write a unit that a court can measure, and they do not agree with each other about which unit that is.

| Jurisdiction | What the statute says | In grams | In ounces | |---|---|---|---| | Colorado — C.R.S. 18-18-406, as amended by HB21-1090 (signed 20 May 2021) | "two ounces" | 56.70 g | 2 | | California — Health and Safety Code § 11362.1(a)(1) | "28.5 grams" | 28.5 g | 1.0053 | | Canada — Cannabis Act, s. 8(1)(a), with Schedule 3 | "30 g of dried cannabis" | 30 g | 1.0582 |

California's figure is the interesting one. Proposition 64 could have written "one ounce" and did not; it wrote 28.5 grams, which is 0.15 g more than an ounce. Anyone treating the state limit and a zip as the same quantity has the direction of the rounding backwards. Canada's 30 g runs further still, at 1.058 ounces, and it is a dried-cannabis equivalent under Schedule 3 rather than a raw weight, so concentrates convert at 0.25 g per gram of dried and count against the same ceiling. Colorado kept the customary unit and doubled it in 2021.

Three jurisdictions, three units, one slang term that maps cleanly onto none of them.

What a pharmacy label and a food label do with the same problem

Every lawful product label solves this by declaring net mass and naming the rule it declares under.

| Label | Governing rule | Unit required | Variation permitted | |---|---|---|---| | Packaged food, US | 21 CFR 101.7, under the Fair Packaging and Labeling Act | Avoirdupois pound and ounce, with metric alongside | Handbook 133 MAVs; the lot average must still meet the label | | Prescription drug, US | 21 CFR 201.51 | Avoirdupois pound, ounce and grain, or kilogram, gram and subdivisions | "Reasonable variations" under good manufacturing practice | | Cannabis, Canada | Cannabis Regulations, s. 133 | Grams | 10 % at or below 2 g, 5 % above |

A cereal box, a bottle of tablets and a jar of flower all declare net mass, all name a unit with a legal definition behind it, and all come with a published statement of how far reality is allowed to sit from the printed number. "Zip" gives you a quantity with no unit definition, no issuing body and no tolerance. It is a useful word among people who already agree what it means, and it carries nothing forward to anyone who does not.

Where the word came from, and how firm that story is

Green's Dictionary of Slang, the standard reference for this kind of vocabulary, has an entry for zip meaning one ounce of a given drug. That much is documented lexicography.

The origin story is softer. The common account is the Ziploc bag: an ounce of flower fills a sandwich-sized bag and leaves enough room to close it, so ounce became zip. The dates do not contradict it. Dow Chemical test-marketed the Ziploc brand in 1968, and the Dow engineer Robert Vergobbi filed the reclosable-bag patent, US 3,515,241, the same year. The plastic zipper itself is older, traced to Flexigrip in 1951 and Minigrip in 1961. The bag was in kitchens for decades before the slang became common.

A competing explanation is the abbreviation: oz contains a z. Both accounts are plausible and neither is documented well enough to settle it.

I could not find a first dated citation for the sense, and I am not going to invent one. The claim I have seen repeated — that the word spread through the 1990s and reached wide recognition by the early 2000s — comes from cannabis-industry glossaries rather than from lexicographers, and I would treat it as folk history until someone produces a dated quotation. For contrast, the Online Etymology Dictionary dates lid for an ounce of marijuana to 1967. That is what firm looks like.

Why I keep every figure attached to its source

I came into aftercare writing through voicemail transcription for dental offices, which meant typing out hundreds of post-extraction callbacks, and early on I did something I still think about. Two clinics had sent instructions for the same procedure. I merged them into one clean summary because the overlap was obvious and the differences looked like noise. They were not noise. One office had a reason for its longer window that the other did not share, and the patient reading my tidy version had no way to tell whose rule she was following.

The correction taught me the discipline this whole article runs on: a number without its source is a rumour with a decimal point.

The smoking question makes it concrete, and it lands on my desk constantly. A 2022 systematic review in Dentistry Journal pooled eleven studies from ten countries covering 10,195 patients and found dry socket in about 13.2 % of smokers against 3.8 % of non-smokers, more than a threefold increase in the odds. That figure is measured, published and checkable. The number patients actually get told — wait 72 hours — is office convention rather than a published national guideline, and offices vary, with some extending to seven or ten days after a surgical extraction. Both numbers are useful. Only one of them has a citation, and you deserve to know which is which before you decide.

Same shape as the zip. Twenty-eight grams is a convention. 28.349523125 is the definition. Ask which one you are being handed.

Questions that arrive with this one

How many grams is a full zip?

A full zip is one avoirdupois ounce, 28.349523125 grams exactly, from the international pound of 0.453 592 37 kilograms divided by sixteen. Everyday use rounds it to 28 grams, giving up 0.35 g. Both figures describe the same quantity; only the long one is the definition, and the short one is convention.

Why is 28 grams called a zip?

The usual explanation is the Ziploc bag, which Dow Chemical test-marketed in 1968 and which holds about an ounce of flower with room to seal. A second account traces it to the z in the abbreviation oz. Green's Dictionary of Slang records the meaning; neither origin story has a firm dated citation behind it.

How much is a zip in ounces?

A zip is exactly one ounce, avoirdupois. That is the ounce used on food and drug labels and in cannabis statutes, defined as 28.349523125 grams. It is not the troy ounce of 31.1034768 grams used for precious metals, and it is not the US fluid ounce, which measures a volume of 29.5735295625 millilitres.

How many grams are in half a zip?

Half a zip is half an avoirdupois ounce: 14.1747615625 grams exactly, commonly quoted as 14 grams. The rounding gives up 0.17 g. A quarter zip is 7.08738078125 grams, usually written as 7 grams, and an eighth is 3.543690390625 grams, usually written as 3.5 grams.

What does zip mean in drug slang?

Green's Dictionary of Slang defines zip as one ounce of a given drug. It is used most often for cannabis flower, where a zip means 28 grams in practice. The term carries no legal definition, no issuing authority and no tolerance, which is why statutes and product labels use grams or ounces instead.

Does a legal limit use grams or slang?

Statutes use measurable units, never slang. California's Health and Safety Code § 11362.1 sets 28.5 grams, which is 0.15 g more than an ounce. Canada's Cannabis Act sets 30 g of dried cannabis. Colorado uses the customary unit, two ounces, or 56.70 grams. Check your own jurisdiction's wording.