Carat Scale for Gemstones and Diamonds: Readability, Capacity and NTEP Class II

A carat scale for gemstones and diamonds should read 0.001 ct to 0.005 ct, and for grading and appraisal we recommend the Sartorius GCL603I-2S (600 ct x 0.001 ct), on sale for $1,568.00 (regular $2,390.00). To sell stones by weight, the NTEP Class II OHAUS SJX323N/E is $738.00; both are in stock.

Reviewed by MNM Scales technicians · Updated November 1, 2026 · Prices checked November 1, 2026

Gem dealers, jewelers, appraisers and graders weigh stones in carats, and the difference between a 0.99 ct and a 1.00 ct stone is a single point. MNM Scales has sold, calibrated and repaired balances for over 30 years from our shop in Cypress, Texas. This guide explains carats versus grams, how much readability and capacity a carat scale for gemstones needs, and when the scale has to be NTEP Class II.

What does CT mean on a scale?

CT on a scale display means carats. The metric carat is exactly 0.2 g (200 mg), so 5 ct equals 1 g. Diamonds are also described in points: 1 ct is 100 points, so one point is 0.01 ct, or 2 mg. Karat with a K is a different measure: it describes gold purity, not weight (see our legal-for-trade jewelry scale guide for gold). Convert between carats, grams and grains with our grain to gram converter.

Carats	Grams	Milligrams	Points
0.001 ct	0.0002 g	0.2 mg	0.1 point
0.005 ct	0.001 g	1 mg	0.5 point
0.01 ct	0.002 g	2 mg	1 point
0.1 ct	0.02 g	20 mg	10 points
1 ct	0.2 g	200 mg	100 points
5 ct	1 g	1,000 mg	500 points
600 ct	120 g	120,000 mg	–

Carat scale for gemstones: readability

Readability is the smallest step the display shows (more in our capacity and readability guide). For gemstones it is usually quoted in carats:

• 0.001 ct (0.2 mg): a tenth of a point. Fits diamond grading, small stones, melee lots and appraisal work. The GCL603I-2S reads to 0.001 ct.
• 0.005 ct (1 mg): half a point. Fits everyday buying and selling of loose stones. The SJX323N/E shows 0.005 ct (0.001 g).
• 0.01 ct (2 mg): one point. Fits larger stones and parcels where a point is fine enough.

For selling, the number that counts is the certified readability, the verification interval e on the NTEP certificate. The SJX323N/E displays 0.001 g, and its certified readability is 0.01 g (0.05 ct). A balance this fine also needs a draft shield, a level, sturdy table away from vents, and a few minutes to warm up.

Capacity: how many carats do you need?

• Loose stones: a 320 ct to 760 ct carat scale holds single stones with room for a parcel paper or tray.
• Parcels and lots: weigh a full parcel at once with 600 ct (GCL603I-2S) or 760 ct (HR-150A).
• Mounted pieces and rough: a ring, pendant or large rough can pass a few hundred carats; check the total against capacity, including any tray.

NTEP Class II for selling gemstones

NIST Handbook 44 lists Class II for laboratory weighing, precious metal and gem weighing, so a scale used to sell stones by weight in the US should carry an NTEP certificate for Class II. The OHAUS SJX323N/E is NTEP certified, Class II (Certificate of Conformance 16-015), and Measurement Canada approved. Use the GCL603I-2S or HR-150A for grading, appraisal and inventory weighing, and the SJX323N/E for weights that set a sale price. Read more in our guide to NTEP and legal-for-trade scales.

Selling stones in Texas: TDA does not require an NTEP certificate for every device, but the scale must meet NIST Handbook 44 and suit its use; some scales do need one, so we recommend NTEP models and can check yours. TDA explains this in Legal for Trade Scales in the State of Texas (PDF). See our TDA scale certification guide for placing a scale in service.

Carat scales in stock

Carat scale	Capacity x readability	Best for	Key features	Price
Sartorius GCL603I-2S	600 ct x 0.001 ct (120 g)	Grading, appraisal, diamond dealers	Internal calibration, draft shield, carat and gram units	$1,568
OHAUS SJX323N/E	320 ct x 0.005 ct (64 g x 0.001 g)	Selling stones by weight	NTEP Class II (e = 0.01 g), draft shield, carat, gram, grain units; AC adapter included, 4 AA batteries not included	$738
A&D HR-150A	760 ct x 0.0005 ct (152 g x 0.1 mg)	Finest readings, large parcels	Replaces the A&D FX-700CT, same 152 g capacity and 90 mm pan, carat display	$1,049

See more models in precision balances.

Getting accurate stone weights

1. Set the balance on a sturdy, level table away from fans, vents and doors, and level it with the bubble.
2. Let it warm up, then calibrate: the GCL603I-2S calibrates itself; the SJX323N/E and HR-150A use a calibration weight.
3. Close the draft shield, tare the parcel paper or tray, and handle stones with tweezers.
4. Wait for the stable indicator before you read the weight.
5. Have the balance calibrated with a certificate on a regular schedule; see balance calibration in Houston or mail-in calibration.

Frequently asked questions

What does CT mean on a scale?

CT means carats. One metric carat is exactly 0.2 g (200 mg), so 5 ct equals 1 g, and 1 ct is 100 points.

How many carats are in a gram?

Five. One gram equals 5 ct, and one carat equals 0.2 g.

What readability do I need to weigh diamonds?

0.001 ct (a tenth of a point) for grading and small stones, or 0.005 ct for everyday buying and selling. For selling, check the certified readability on the NTEP certificate.

What scale should I use to sell gemstones by weight?

An NTEP Class II scale, such as the OHAUS SJX323N/E ($738), which is NTEP certified Class II with a certified readability of 0.01 g (0.05 ct).

What is the best carat scale for gemstones?

For grading and appraisal, the Sartorius GCL603I-2S ($1,568) reads 600 ct x 0.001 ct with internal calibration. For selling by weight, the NTEP Class II OHAUS SJX323N/E ($738).

What replaced the A&D FX-700CT carat balance?

The A&D HR-150A ($1,049), with the same 152 g (760 ct) capacity and 90 mm pan, a carat display and 0.1 mg (0.0005 ct) readability.