Unit Conversion

Precious-metal markets use the troy system rather than the everyday avoirdupois system. To avoid ambiguity, this page documents the exact mass of each unit we support, shows the relationship to troy ounces (oz t), and links to primary/authoritative references where available. Our calculators normalize every input to grams and troy ounces using these standards.

1) The troy system (gold, silver, platinum, etc.)

The troy system is the global convention for bullion. In the troy system: 20 pennyweights (dwt) = 1 troy ounce (oz t) = 480 grains (gr); 12 troy ounces = 1 troy pound. The exact modern conversion is 1 oz t = 31.103 476 8 g. These relations are published by the U.S. National Institute of Standards and Technology (NIST) in Handbook 44, Appendix C and in NIST’s precious-metals conversion guidance. See: NIST HB-44 (2024), App. C and NIST OWM: Conversion Factors for Precious Metal Sales (2015).

• Troy ounce (oz t) — exactly 31.103 476 8 g (NIST HB-44).
• Pennyweight (dwt) — exactly 1/20 oz t = 1.555 173 84 g (NIST HB-44).
• Grain (gr) — exactly 1/480 oz t = 0.064 798 91 g (NIST HB-44).
• Troy pound (lb t) — 12 oz t (note: not used in day-to-day bullion trading; listed for completeness) (NIST HB-44).

2) Metric and gem trade units

The bullion world also quotes weights in grams and kilograms, and the gem trade uses the metric carat. We follow the SI and gem-standards below:

• Gram (g) — SI base unit of mass.
• Kilogram (kg) — 1,000 grams (SI).
• Carat (ct) — exactly 200 mg (0.2 g), the metric carat, adopted internationally in 1907 and used by GIA and the global gem trade. See: GIA: Diamond Carat Weight and Carat (mass) background.

3) Regional bullion/jewelry units (standardized, with sources)

3.1 Tola (South Asia)

The tola is a traditional South Asian unit still common in retail gold trade and bar denominations (e.g., 10-tola bars). Under British India the tola was standardized at 180 troy grains, which equals 11.663 803 8 grams or exactly 3/8 oz t = 0.375 oz t. See: Tola (historical overview). Modern practice in India, Pakistan, Bangladesh, Nepal and elsewhere uses this 180-grain standard for bullion bars.

3.2 Baht (Thailand)

In Thailand, baht is both the currency and a gold-weight unit. For bullion, 1 baht = 15.244 g ≈ 0.4901 oz t. This is the standard quoted by the Singapore Bullion Market Association’s review of Thailand’s gold market, and by the Thai Gold Traders Association. See: SBMA: Thailand’s Gold Market and GoldBarsWorldwide: Gold Conversion Tables (lists 15.244 g and 0.4901 oz t). Note: for jewelry manufacturing in Thailand, a working value of ~15.16 g is sometimes referenced, but bullion standardization uses 15.244 g.

3.3 Tael (liang)

Hong Kong tael (tael troy): Hong Kong law recognizes a tael (troy) used in the bullion trade of 37.429 g ≈ 1.20337 oz t. See the Weights and Measures legislation: Hong Kong Weights & Measures Order – Schedule (conversion table).

Japan/Taiwan variant (37.5 g): Historically, Japan used the momme (匁), fixed in the Meiji era at 3.75 g. Ten momme equals one ryō/tael, i.e., 37.5 g ≈ 1.20597 oz t. Modern Japanese bullion trade is metric, but this 37.5 g “tael” persists in regional usage (and is standard for Taiwanese tael in some markets). See: Nippon.com: traditional units (momme = 3.75 g) and GoldBarsWorldwide (notes a 37.50 g tael variant).

3.4 Vori/Bori (Bangladesh usage)

In Bangladesh, the retail jewelry trade commonly uses vori/bori as synonyms for the standardized tola. Practical conversion tables used locally define 1 vori = 1 tola = 11.664 g, with sub-units (ana, roti/rotti, point). See: GoldBangladesh: Gold Weight Conversion (Bangladesh Standard). We treat vori/bori as aliases of tola (180 grains) in calculators unless a page explicitly states otherwise.

3.5 Mithqāl / Mesghal

The term mithqāl has multiple historical definitions. A widely cited modern standard in Iran is the mesghal at 4.6083 g (≈ 0.14801 oz t), referenced by the Central Bank of Iran via industry tables. See: GoldBarsWorldwide: Gold Conversion Tables. Classical Islamic texts and some South Asian religious/fiqh contexts also reference a mithqāl close to 4.25 g; because this value is not uniform across jurisdictions, we always label the variant when using mithqāl in content.

3.6 Ratti and Masha (jeweler’s traditional units)

The ratti derives from the Abrus precatorius seed and is part of an ancient ladder of jeweler’s weights. A common jeweler’s chain is 8 ratti = 1 masha, 12 masha = 1 tola. If tola is 11.6638 g, this implies 1 ratti ≈ 0.1215 g and 1 masha ≈ 0.972 g. Documented overviews: Ratti (unit) and Masha (unit). Because ratti and masha have regional variants (e.g., “metric ratti” in some gem markets), we show the exact factor used alongside calculators where applicable.

4) Authoritative conversion sources we rely on

• NIST Handbook 44, Appendix C — official U.S. reference for troy/metric factors: 2024 edition (PDF).
• NIST OWM precious-metals guide — practical oz t/gram and dwt conversions for trade: PDF.
• Hong Kong tael (troy) — Hong Kong e-Legislation conversion table: Cap. 68A, Schedule.
• Baht (Thailand) — SBMA industry article and conversion tables: SBMA, GoldBarsWorldwide.
• Tael 37.5 g variant — historic momme and regional tael: Nippon.com (momme = 3.75 g), GoldBarsWorldwide (37.50 g note).
• Carat (ct) — GIA education: GIA.
• Vori/Bori (Bangladesh) — practical local table: GoldBangladesh.

5) Quick reference table (grams and troy ounces)

Values below reflect the standardized definitions cited above. Where multiple regional standards exist, we either list the regional row explicitly or document the variation in the notes.

Important: The everyday avoirdupois ounce (28.3495 g) and pound (16 oz avdp) are not used for bullion pricing. All metals data on this site refer to troy units unless explicitly labeled.

6) Worked examples (how we compute)

1. Convert 3 baht (Thailand) to troy ounces
   Use the bullion standard 1 baht = 15.244 g. 3 × 15.244 = 45.732 g. Divide by the exact troy-ounce gram value: 45.732 ÷ 31.1034768 = 1.4703 oz t. (SBMA; NIST HB-44)
2. Convert 1 Hong Kong tael to grams
   By statute, 1 HK tael (troy) = 37.429 g (HK Cap. 68A).
3. Compare Hong Kong tael vs Japan/Taiwan tael
   HK tael = 37.429 g ≈ 1.20337 oz t; Japan/Taiwan tael = 37.5 g ≈ 1.20597 oz t. The Japan/Taiwan variant is heavier by ~0.071 g (~0.0026 oz t). (HK e-Legislation; Nippon.com; GBW)
4. From ratti to grams (jeweler’s chain)
   If 12 masha = 1 tola and 8 ratti = 1 masha, then 96 ratti = 1 tola. With tola fixed at 11.6638038 g, 1 ratti = 11.6638038 / 96 = 0.121498 g.

7) Data quality and regional variations

Several traditional units have coexisting definitions across regions and eras. Our policy:

• Prefer primary/official sources (e.g., NIST, statutes, recognized industry bodies) when available.
• Document and label variants (e.g., HK tael vs Japan/Taiwan 37.5 g; bullion vs jewelry baht).
• Expose the exact factor next to calculators when a unit has multiple accepted values in commerce.

If you see a different factor in a local shop or on an invoice, check which variant that source uses. Our conversion widgets will show the factor in use on the page (hover the ⓘ icon next to the unit).

8) Notes for developers (consistency with calculators)

• Troy constants: keep OZT_TO_G = 31.1034768 (double precision), and derive DWT_TO_G = OZT_TO_G / 20, GRAIN_TO_G = OZT_TO_G / 480.
• Tola: use 180-grain standard (11.6638038 g; 0.375 oz t).
• Baht: for bullion use 15.244 g; expose a toggle/note if a jeweler uses 15.16 g for workmanship norms.
• Tael: default HK tael (37.429 g). Where a page targets Japan/Taiwan historical content, allow the 37.5 g variant and label it.
• Mithqāl: if used, require the variant flag (mithqal=mesghal_4_6083 or mithqal=classical_4_25) to avoid silent mismatches.
• Ratti/Masha/Vori: always show the factor inline near the input.

9) Sources & Further Reading

• NIST Handbook 44 (2024), Appendix C — PDF
• NIST OWM, Conversion Factors for Precious Metal Sales (2015) — PDF
• Hong Kong Weights & Measures (Cap. 68A), Schedule (Tael, etc.) — Link
• SBMA, Thailand’s Gold Market — Link
• GoldBarsWorldwide, Gold Conversion Tables — PDF
• Nippon.com, Traditional Japanese Units (momme = 3.75 g) — Link
• GIA, Diamond Carat Weight — Link
• Vori/Bori (Bangladesh retail table) — Link
• Background: Troy weight — Overview
• Background: Tola (historical) — Overview
• Background: Ratti — Overview
• Background: Masha — Overview

Note: Regional measurement practices may differ by country and historical period. For transparency, we strive to show the specific conversion factors (e.g., reference weights or mass values) used on each relevant page. All information is provided for general reference only and use of this site constitutes acceptance of our Disclaimer and Privacy Policy.
Related: Methodology Hub.

Last Updated: 15 November 2025