Pistachio basics
What Is a Pistachio? The Nut, the Word and the US Grades
The green seed of a desert drupe. We put both USDA pistachio grade standards side by side, and nearly everything separating the top grades is shell rather than nut.
Quick answer
A pistachio is the edible green seed of Pistacia vera, a desert tree in the cashew family. The seed sits inside a stone, and the stone sits inside a fleshy fruit called a drupe, so botanically it is a seed rather than a true nut, though food labels and allergy law treat it as a tree nut. English took the word from Italian pistacchio in the 1590s, and it traces back through Latin and Greek to an Iranian source. In the United States, in-shell pistachios carry one of six USDA grades and shelled kernels one of three.
A pistachio is the green seed of Pistacia vera, a small desert tree in the cashew and mango family. Three layers stand between the tree and your hand. The fruit is a drupe, so a soft hull grows around a hard stone, and the seed sits inside the stone. Hulling strips the outer layer at harvest. What reaches a bag is the stone, split along its seam, with the seed inside.
That split is the part everyone recognizes, and it happens on the tree as the seed swells. It is a sign of ripeness rather than a crack.
Is a pistachio a nut, a seed or a fruit?
Each answer is right about something different. The tree's fruit is a drupe, the same build as a cherry, a peach or an olive. The pistachio you eat is the seed inside that drupe's stone, which makes it botanically a seed and not a true nut. In the kitchen, and on every US allergen label, it is a tree nut. The longer version, including why almonds sit in the same odd category, is on are pistachios nuts or seeds.
The green is chlorophyll, with yellow carotenoids under it. Most seeds lose their chlorophyll as they dry, which is why an almond is beige; the pistachio keeps enough of its own to carry the color into a gelato. That has its own page: why pistachios are green.
Where does the word pistachio come from?
English took it from Italian pistacchio in the 1590s. Behind the Italian sits Latin pistacium, and behind that Ancient Greek pistákion, the nut of the pistákē tree, which Greek had borrowed from an Iranian language. Merriam-Webster names Persian pistah as a relative. The same root gave Persian peste, Arabic fustuq and Turkish fıstık.
There was an earlier English attempt. Pystace and pistace appear in the mid fifteenth century, borrowed from Old French pistace and Medieval Latin pistacia, and they lost out to the Italian shape a century and a half later.
So how do you spell it? One c, then ch. The double c belongs to Italian, which keeps pistacchio, and borrowing the word from Italian is what makes the English spelling catch people out. Pistacchio is right in Italian and wrong in English. Pistachio is right in English.
How are pistachios graded in the US?
By the shell, mostly. USDA runs two separate standards, one for pistachios in the shell and one for shelled kernels, and both still carry an effective date of 23 December 2004. The in-shell standard defines six grades; the shelled standard defines three. We compiled the two into one table on 31 August 2026, because the difference between them is invisible while each sits in its own PDF.
Downloadable as a CSV file, free to reuse with attribution (see citing this page).
| Grade | Form | Shells not properly split | Light-stained shells | Kernel damage | Serious kernel damage | Insect damage |
|---|---|---|---|---|---|---|
| U.S. Fancy | In-shell | 2% | 7% | 3% | 3% | 1% |
| U.S. Extra No. 1 | In-shell | 3% | 12% | 6% | 4% | 2% |
| U.S. No. 1 | In-shell | 6% | 25% | 6% | 4% | 2% |
| U.S. Select | In-shell | 10% | No limit set | 6% | 4% | 2% |
| U.S. Artificially Opened | In-shell | 10% | No limit set | 6% | 4% | 2% |
| U.S. Non-Split | In-shell | Not applicable | No limit set | 6% | 4% | 2% |
| U.S. Fancy | Shelled | Not applicable | Not applicable | 2.0% | 1.5% | 0.3% |
| U.S. Extra No. 1 | Shelled | Not applicable | Not applicable | 2.5% | 2.0% | 0.4% |
| U.S. No. 1 | Shelled | Not applicable | Not applicable | 3.0% | 2.5% | 0.5% |
All figures are tolerances by weight: the most of that defect a lot may contain and still carry the grade. "Shells not properly split" is the standard's combined row for non-split shells and shells split somewhere other than the suture. The full matrix, including adhering hull, dark stain, undersized nuts, blanks, loose kernels and dust, is in the CSV.
Putting them together shows three things.
The grade ladder is a shell ladder. Going from U.S. Fancy down to U.S. No. 1 raises the light-stain tolerance from 7% to 25%, more than three times as much staining on a part nobody eats. Over the same three grades the kernel-damage tolerance only doubles, 3% to 6%, and total internal defects go from 4% to 9%. The number that moves furthest between grades describes the shell.
Note also that the six in-shell grades are not one clean ladder. Fancy, Extra No. 1, No. 1 and Select do step down on shell appearance. U.S. Artificially Opened and U.S. Non-Split describe different products: nuts opened mechanically after harvest, and nuts that never opened at all, which mostly go on to be shelled.
The same grade name promises more once the shell is off. U.S. Fancy in the shell allows 3% kernel damage and 1% insect damage. U.S. Fancy shelled allows 2.0% and 0.3%. The two sets are not measured on the same base, since the in-shell figures are a share of whole nuts including shell while the shelled figures are a share of kernels alone. Correcting for that widens the gap rather than closing it, because taking shell weight out of the denominator can only raise the in-shell percentage. Either way, a bag of kernels graded U.S. Fancy is a stricter promise about the nut than a bag of in-shell nuts wearing the same words.
Neither standard grades a pistachio on how it tastes, with one narrow exception. The only defect defined by taste is rancidity, "the kernel is distinctly rancid to taste", followed at once by the instruction that staleness of flavor shall not be counted as rancidity. Flavor is mentioned once more in passing, where an immature shelled kernel is one whose immaturity "has adversely affected the flavor". Kernel color turns up in that same definition and nowhere else, as a defect: such a kernel "can have black, brown or gray surface with a dark interior color". Nothing anywhere rewards a pistachio for being green or for being sweet.
What do the size names mean?
They are defined counts. The in-shell standard sets five size designations by the average number of nuts per ounce, counted before roasting.
| Size designation | Average nuts per ounce | Weight per nut at that count |
|---|---|---|
| Colossal | Fewer than 18 | Over 1.58 g |
| Extra Large | 18 to 20 | 1.42 to 1.58 g |
| Large | 21 to 25 | 1.13 to 1.35 g |
| Medium | 26 to 30 | 0.95 to 1.09 g |
| Small | More than 30 | Under 0.95 g |
The weight column is our arithmetic, using the standard's own conversion of one ounce to 28.35 grams, rounded to two decimals. At the boundaries a Colossal nut weighs at least two-thirds more than a Small one. Below all of it there is a floor: any nut that falls through a 30/64 inch round hole screen counts as undersized, and no grade allows more than 5% of those.
Shelled kernels are sized differently again, by how much of the sample stays whole and what passes through a screen. Jumbo Whole Kernels and Large Whole Kernels both require 80% or more whole kernels by weight; Large Split Kernels requires 75% halves split lengthwise. Those four classes are in a second CSV.
A blank is a legal term
The standard defines a blank as a shell that never split and holds either no kernel at all or a kernel filling less than half the cavity. Every in-shell grade caps blanks at 1% of the lot.
That 1% is worth holding next to what happened in California in 2026, when growers reported blank rates as high as 80% after an early bloom met record heat. The two numbers measure different moments: the field figure describes the crop on the tree, the tolerance describes a lot that has already been sorted. We track what that year did to prices on the pistachio price index, and the reason the trees bloomed out of step is on the pistachio tree.
What does a pistachio taste like?
Mild, faintly sweet and buttery, with far less bitterness than a walnut and less of the sharp almond edge. Roasting brings out a toasted, slightly savory note; salt pushes it further that way. Raw kernels taste greener and softer, closer to a fresh pea than most people expect, which is why they suit desserts and cold dishes better than roasted ones do. The comparison of the two is on roasted vs raw pistachios, and the flavor as it carries into a drink is on what pistachio milk tastes like.
Origin changes it too. Iranian cultivars run longer and more intensely flavored, Californian ones rounder and milder, and the two are set against each other on Iranian vs American pistachios.
What forms are pistachios sold in?
- In the shell, raw, roasted or salted, and occasionally dyed red, a habit left over from mid-century imports.
- Shelled kernels, whole or split, for baking and cooking. Getting them out of the shell yourself is covered on how to shell pistachios.
- Ground, as flour or pistachio powder.
- Pistachio paste and pistachio butter, which are the nut alone, and pistachio cream, which is usually sweetened and often holds more sugar than nut.
- Pistachio milk, blended and strained. To make it, see how to make pistachio milk.
Which cultivar you are eating changes the shape and the color as much as the processing does: see types of pistachios. For the countries that grow them, where pistachios grow.
The health question has its own answer and its own dataset. USDA puts raw pistachios at 560 kcal, 20.16 g of protein and 10.6 g of fiber per 100 g, which is a nutritious food by any reading. The trial evidence behind the health claims is thinner than it looks: 30 published papers report only 21 separate feeding trials, and the ones with a fixed dose fed a median of 56 g a day, twice the serving on the pack. The whole register is on are pistachios good for you.
Methodology, limitations and corrections
We compiled these tables on 31 August 2026 from the two current USDA Agricultural Marketing Service grade standards for pistachios, read as published rather than as summarized elsewhere.
What we included. Every defect tolerance in 51.2544 Tables I, II and III (in-shell) and 51.2557 Table 1 (shelled), every size designation in 51.2546 Table IV, and the four kernel size classifications in 51.2559. What we excluded: the AMS inspection instructions, which explain how to run the test rather than what the standard requires; the separate AMS commodity specifications used for federal purchasing, which are procurement documents and not grades; and the California marketing order, which is a different instrument again.
The limits, plainly:
- This is a compilation of published rules, not a test of any product. We inspected no nuts. Every figure is what the federal standard permits, not what a given bag contains.
- The in-shell and shelled tolerances share names but not a denominator. In-shell figures are a percentage by weight of whole nuts, shell included; shelled figures are a percentage by weight of kernels. We have said so beside the comparison rather than converting between them, because the shell fraction varies by cultivar and by season.
- "No limit set" is not the same as "anything goes". U.S. Select, U.S. Artificially Opened and U.S. Non-Split carry no ceiling on light staining, but dark staining is still capped at 3% in all three. The standard prints "N/A" in those cells and we have not read more into it.
- The weight-per-nut column is derived, not published. USDA gives counts per ounce; we divided using the metric conversion table printed in the shelled standard. The counts are averages for a lot, so no individual nut is guaranteed to fall in the range.
- A grade is voluntary. These are grade standards used when a lot is inspected and certified. A bag on a US shelf carrying no grade mark has not failed anything; it has simply not been graded.
- The standards are old. Both carry an effective date of 23 December 2004. If AMS revises either, this page will be wrong until we update it, so check the source documents before quoting the figures for anything that matters.
Spot an error, or know of a revision we have missed? Email us via the about page and we will correct it and note the change here.
Citing this page
The compiled tables and datasets are original work. You're welcome to reuse them, including for commercial publication, with attribution to pistachiomilk.org and a link to this page (Creative Commons CC BY 4.0). A suggested citation:
Ricci, E. (2026). What Is a Pistachio? The Nut, the Word and the US Grades. pistachiomilk.org. https://pistachiomilk.org/what-is-pistachio/
Version history: v1.0, 31 August 2026. First publication of the compiled USDA grade comparison (9 grade rows across both standards, 114 tolerance figures in the dataset) and the size designation table. Corrections and additions will be listed here.
Allergy note
Pistachios are a tree nut. The botany does not change the advice: anyone with a tree nut allergy should avoid pistachios and anything made from them, including pistachio milk. The cashew relationship is the one to know about, and the nut is not toxic in any other sense: are pistachios poisonous covers both, along with the legal aflatoxin limits that are the only contaminant figures set for this crop.