How to Choose a Jewelry Scale: Carat Mode, Repeatability and Legal-for-Trade Rules

Quick answer: choosing a scale for jewelry work

  • First decide: bench use or selling by weight? In most US states (44 of 50 are NTEP states, per Bench Jeweler), a scale used to buy or sell by weight must be NTEP-approved. Some states add more: Michigan, for gold, also requires an on-site test before use. Check with your state weights and measures office.
  • Carat mode matters for stones. One metric carat is exactly 200 mg (0.2 g). Make sure "ct" is available on the model you buy, and know how to switch it on.
  • Look at repeatability, not just the display. A 0.001 g display does not mean every reading repeats to 0.001 g. Check the repeatability figure in the spec sheet.
  • Capacity: choose for your heaviest single item plus any dish or tray.
  • Check how it is calibrated: the A&D EJ-303B, for example, uses an external weight sold separately.

"Jewelry scale" covers everything from pocket scales to lab balances. This guide sorts out the questions that actually change which scale you should buy: what you will weigh, whether the weight is used for a sale, which units you need, and how precise the scale really is. Technical values come from A&D's documents, official sources and our product listing, all noted below.

1. Bench use or trade use? Decide this first

The first question to settle is not resolution. It is whether the scale needs to be approved for trade.

  • Trade use means the weight sets a price: buying scrap gold, selling loose stones by the carat, or charging by weight at the counter. The State of Michigan, for example, requires scales used to buy or sell gold by weight to be NTEP-approved, Class II or Class III, and tested on site before use by a registered agency or a state official.
  • Bench use means the weight helps you work but is not the basis of a sale: checking casting grain, weighing findings, recording a stone before setting, or counting small parts.

NTEP (National Type Evaluation Program) is run by the National Conference on Weights and Measures and checks devices against NIST Handbook 44. Requirements differ by state (six states use their own standards instead of NTEP, per Bench Jeweler), so confirm with your state weights and measures office before you buy a scale for trade.

The A&D balance we stock is a Japanese-market model. Its NTEP approval is Not confirmed, so treat it as a bench and workshop balance, not as a scale for selling by weight in the US.

2. Units: carat, gram and the rest

Carat (ct) is a fixed mass

The metric carat used for diamonds and other gemstones is 200 milligrams. In 1913 the US Bureau of Standards (now NIST) adopted the international metric carat of 200 mg as the unit of weight for diamonds and other precious stones. Do not confuse it with karat, which describes gold purity, not weight.

Check which units your exact model can show

The same model family can differ by market. A&D's EJ series brochure lists g, oz, ozt, ct, mom, dwt, gr and N for the international EJ-303. The Japanese manual for the domestic EJ-303B lists only five display modes: g, pcs (counting), % (percent), ct and mom. Pennyweight (dwt) and troy ounce (ozt) are not in that list.

On the EJ-303B, only g, pcs and % are registered at the factory. You switch on ct (and mom, used for pearls) with the unit registration procedure in the manual. The manual also notes that the carat is a unit only for gemstones.

3. Readability vs repeatability

Readability (minimum display) is the smallest step on the screen. Repeatability is how closely repeated weighings of the same item agree. For a 0.001 g balance, these are not the same number.

Example from A&D's manual for the EJ-303B: minimum display 0.001 g, repeatability (standard deviation) 0.003 g, linearity ±0.003 g. When you compare scales, put the repeatability and linearity figures side by side, not only the display resolution.

Check the carat figures too. A&D's manual lists a carat capacity of 1550 ct and a minimum display of 0.005 ct for the EJ-303B.

4. Capacity and pan size

Think about the heaviest single thing you will weigh, plus any container or tray. A small round pan is easy to clean, but a stone dish or tray takes up part of the capacity.

The EJ-303B has a 310 g capacity and a φ110 mm pan, according to A&D's manual and our product listing.

5. Calibration and accuracy over time

  • External weight: A&D's manual describes calibrating the EJ-303B with a weight sold separately. The factory calibration value is 300 g, and the manual says to use a weight of at least 2/3 of the capacity.
  • Gravity correction: the manual offers gravity correction as an alternative when you have no weight. It is not needed if you calibrate with a weight where the balance is used.
  • Temperature: A&D's brochure gives sensitivity drift of ±20 ppm/°C (10 to 30 °C) for the EJ series. The manual says to power the balance on with an empty pan for at least 30 minutes before calibrating.
  • Certificate: our listing for the EJ303B-JA-00J00 states that a JCSS calibration certificate is included. It is not the same as US NTEP approval.

6. Power, counting and data output

  • Power: the EJ303B runs on the included AC adapter or 4 AA batteries (sold separately), per A&D's manual and our listing.
  • Counting: counting mode uses sample sizes of 5, 10, 25, 50 or 100 pieces, per A&D's manual.
  • Percent mode: shows weight as a percentage of a reference, per the manual.
  • Data output: optional USB (EJ-02) or RS-232C (EJ-03) interface. The manual says the two cannot be installed at the same time, and the USB option sends weight values only.

Spec summary: A&D EJ-303B

Item Value Source
Capacity 310 g A&D manual, our listing
Minimum display 0.001 g A&D manual, our listing
Repeatability (std. dev.) 0.003 g A&D manual
Linearity ±0.003 g A&D manual
Display modes g, pcs, %, ct, mom (ct and mom need registering) A&D manual
Carat capacity × minimum display 1550 ct × 0.005 ct A&D manual
Calibration External weight (sold separately), factory value 300 g A&D manual
Pan φ110 mm A&D manual, our listing
Power AC adapter (included) or 4 × AA (sold separately) A&D manual, our listing
Interface Optional USB (EJ-02) or RS-232C (EJ-03) A&D manual
Certificate JCSS calibration certificate included Our listing
NTEP (US legal for trade) Not confirmed –
Country of origin South Korea Our listing
A&D EJ303B compact balance 310 g x 0.001 g Bench balance, 0.001 g A&D EJ303B-JA-00J00

310 g × 0.001 g, carat mode available after registration, JCSS certificate included. For bench use, not for selling by weight.

View EJ303B

Checklist before you buy

  1. Will the weight set a price? If yes, look for an NTEP-approved scale and confirm local rules.
  2. Is ct (and dwt, if you work in pennyweights) available on the exact model and market version?
  3. What are the repeatability and linearity, not just the minimum display?
  4. Is the capacity enough for your heaviest item plus a dish?
  5. How is it calibrated, and do you need to buy a weight?
  6. AC only, or batteries too?
  7. Do you need data output to a PC?

FAQ

How many grams is one carat?

One metric carat is 200 mg, which is 0.2 g. This is the international unit for diamonds and other gemstones.

Can I use any 0.001 g scale to sell gold or stones by weight?

Usually not. In NTEP states a trade scale must be NTEP-approved, and Michigan, for gold, also requires a Class II or III scale tested on site before use. Check your state rules.

Does the A&D EJ303B show carats?

Yes. A&D's manual lists ct as a display mode for the EJ-303B (1550 ct capacity, 0.005 ct minimum display), but it is not registered at the factory. You turn it on with the unit registration procedure in the manual.

Does the EJ303B show pennyweight (dwt)?

The Japanese manual for the EJ-303B lists g, pcs, %, ct and mom only. dwt is not in that list. The international EJ-303 lists dwt in A&D's brochure.

Is "carat" the same as "karat"?

No. Carat (ct) is a unit of mass for gemstones. Karat describes the purity of gold.

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Sources

Last reviewed: October 2026.

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