Tank Specific Gravity vs Density vs API Gravity: When Each Matters for Tank Sizing

Three different "weight per volume" metrics show up across industrial tank specs: specific gravity (the polyethylene tank world's lingua franca), density (the engineer's lingua franca), and API gravity (the petroleum world's lingua franca). They measure related properties but differ in reference temperature, units, and inversion of scale. A buyer who confuses 35 deg API crude with a 1.0 SG fluid will spec the wrong tank. This guide unpacks all three metrics, the conversions between them, and shows where each one shows up in real tank-purchase decisions.

OneSource Plastics ships polyethylene rotomolded tanks where SG appears on every datasheet, IBC totes and cubetainers shipping product-specific liquids that may be quoted in density or API gravity, and partners with Specialty & Metal Fabrication for petroleum-service steel tanks where API gravity dominates the technical conversation.

Definitions: The Three Metrics

Specific Gravity (SG)

Dimensionless ratio of fluid density to reference fluid density, both at specified temperatures. ASTM D1429 (Standard Test Methods for Specific Gravity of Water and Brine) and ASTM D854 (Specific Gravity of Soil Solids) both reference water at 4 deg C (1.000 g/cm3) as the standard.

Reference: water at 4 deg C = 1.000 SG (peak density of water).
Industrial-tank convention: often quoted at 73 deg F (22.8 deg C) per ASTM D1998.
Range commonly seen: 0.7 (gasoline) to 2.0 (concentrated sulfuric acid).
Why it matters for tanks: tank wall thickness math is hoop-stress driven; hoop stress is proportional to fluid weight, hence SG.

Density

Mass per unit volume. SI units g/cm3 or kg/m3; US customary units lb/ft3 or lb/gal.

Engineering reference: water density 0.99820 g/cm3 at 20 deg C, 0.99707 at 25 deg C, 1.00000 at 4 deg C.
Conversion: 1 g/cm3 = 62.428 lb/ft3 = 8.345 lb/US gallon.
Why it matters: structural load calculations (foundation, support, plumbing supports) use lb or kg; density gives the direct load number.

API Gravity (Degrees API)

Inverted-scale specific-gravity metric used by the petroleum industry. Per API standard:

deg API = (141.5 / SG) - 131.5

Reference: water = 10 deg API; lighter than water = higher deg API; heavier than water = lower deg API.
Light crude oil: 30-45 deg API (SG ~0.80-0.88).
Heavy crude oil: 10-22 deg API (SG ~0.92-1.00).
Bitumen / asphalt: below 10 deg API (SG >1.00).
Gasoline: 50-70 deg API.
Diesel #2: 30-39 deg API.
Why it matters: petroleum trade contracts, custody-transfer measurement, pipeline tariff schedules all quote API gravity. Tank gauging tables (API MPMS Chapter 11) reference API gravity.

The Quick Conversion Reference

Specific Gravity	Density (lb/gal)	deg API	Common product
0.65	5.42	86.2	Pentane
0.72	6.01	65.0	Gasoline (avg)
0.80	6.68	45.4	Jet fuel A
0.85	7.09	35.0	Diesel #2 / light crude
0.92	7.68	22.3	Heavy crude oil
1.00	8.345	10.0	Water
1.09	9.10	-1.6	Brine 12% NaCl
1.20	10.01	-13.6	Sodium hypochlorite 12.5%
1.34	11.18	-25.9	Hydrochloric acid 35%
1.42	11.85	-32.1	Nitric acid 70%
1.53	12.77	-39.0	Caustic soda 50%
1.69	14.10	-47.8	Ferric chloride 40%
1.84	15.36	-54.8	Sulfuric acid 96-98%

When Each Metric Matters

SG dominates: Polyethylene tank selection

Norwesco, Snyder, Chem-Tainer, Enduraplas, Bushman datasheets list a maximum SG rating per tank. Norwesco MPN 41464 (100 gallon vertical) is rated for 1.5 SG service. Snyder MPN 1500VLPC (1,500 gallon) is rated 1.9 SG. Snyder MPN 2500XLPDS (2,500 gallon XLPE double-rib) is rated 1.9 SG with chemistry compatibility. The SG number drives tank wall thickness, fitting reinforcement, and design margin.

Selection rule: pick tank SG rating equal to or greater than chemistry SG, with margin for temperature derating (see our operating temperature engineering guide for the derating curves).

Density dominates: Foundation and structural design

Pad engineering needs total weight, not SG ratio. A 5,000-gallon water tank holds 41,725 lb of water (5,000 gal x 8.345 lb/gal). The same tank holding 1.5 SG ferric sulfate holds 62,587 lb. Foundation design uses density-derived weight directly. ACI 318 (concrete pad) and IBC Section 1808 (foundation) reference dead-load in lb/ft2; convert SG to density to lb/ft2 for the calculation.

API gravity dominates: Petroleum custody transfer + pipeline

Crude oil sales contract specifies API gravity at 60 deg F. API MPMS (Manual of Petroleum Measurement Standards) Chapter 11.1 (Volume Correction Factors) is the bible for converting between API gravity and observed-temperature volume. Steel petroleum storage tanks (API 650, API 12B, API 12D, API 12F) reference API gravity in custody transfer.

If your business is bulk petroleum receiving / blending / shipping: API gravity is the metric you'll see on every BOL, every meter ticket, every tariff schedule. Converting to SG for tank-sizing math is a one-time exercise; the operational metric remains deg API.

Density dominates: Pump and piping hydraulics

Hydraulic head loss math (Darcy-Weisbach, Hazen-Williams) uses fluid density directly. Pump TDH calculations, pipe pressure drop, and centrifugal pump curves all reference density. SG works as an input but most pump engineers default to density (lb/ft3 or kg/m3).

Real Tank-Sizing Decisions

Decision 1: Sodium hypochlorite 12.5% storage

Chemistry: 12.5% NaOCl, SG 1.20, 10.01 lb/gal, density 74.9 lb/ft3.

Tank rating needed: 1.5 SG minimum, 1.9 SG preferred for chemistry margin.
Material: XLPE strongly preferred; HDPE acceptable but reduced life. Snyder MPN 1500XLPDS or comparable XLPE vertical from Chem-Tainer/Enduraplas.
Foundation: 1,500 gallons x 10.01 lb/gal = 15,015 lb fluid. Add ~600 lb tank weight, total ~15,600 lb. Spread on 64-inch diameter pad = ~22 sq ft = ~700 lb/sq ft. Standard 4-inch reinforced concrete pad handles this.

Decision 2: Diesel #2 storage for fleet shop

Chemistry: Diesel #2, 35 deg API, SG 0.85, 7.09 lb/gal.

Tank choice: petroleum-service tank required. Polyethylene is NOT compatible long-term with diesel; UL-142 single-wall steel or UL-2085 protected aboveground tank.
SG conversion: if a polyethylene utility tank is being considered for off-road dyed diesel only (short-term, ag use), SG 0.85 is well within polyethylene capacity. The chemistry compatibility, not the SG, is the limiter.
Tank gauging: use API MPMS volume correction; diesel volume changes ~0.05% per deg F.

Decision 3: 50% caustic soda (NaOH) bulk storage

Chemistry: 50% NaOH, SG 1.53 at 73 deg F, 12.77 lb/gal.

Tank rating: 1.9 SG XLPE required; 50% NaOH is also a hot service (typical 100-130 deg F to prevent crystallization at room temperature where the freeze point is 53 deg F for 50% solution).
Material: Snyder XLPE 6,000-gallon double-wall is a typical install for water-treatment plant bulk caustic.
Foundation: 6,000 gallons x 12.77 lb/gal = 76,620 lb fluid. Add 1,800 lb tank, ~78,500 lb total. Pad must be engineered, not commodity.
Insulation + heat trace: required to keep 50% NaOH liquid in cold climates.

Decision 4: 96-98% sulfuric acid storage

Chemistry: 98% H2SO4, SG 1.84, 15.36 lb/gal, density 115 lb/ft3.

Tank rating: 1.9 SG XLPE OR rubber-lined / acid-brick steel. 98% sulfuric is a unique case where polyethylene works (the concentrated acid is dehydrated and does not attack PE the way dilute does).
Material: consult OEM compatibility chart; some recommend PE up to 95% only; above 95% switch to lined steel.
Foundation: 4,000 gallons x 15.36 lb/gal = 61,440 lb fluid. Engineered pad with secondary containment (110% of tank volume) per EPA 40 CFR 264.193.

Decision 5: Bulk crude oil receiving (lease tank battery)

Chemistry: light sweet crude, 38 deg API, SG 0.835.

Tank choice: API 12F bolted-steel lease tank, 100-2,000 BBL capacity. Polyethylene NOT applicable.
Custody transfer: volume corrected per API MPMS Chapter 11.1 to 60 deg F reference. Tank gauging per API MPMS Chapter 3.1A.
Specialty & Metal Fabrication: partner network handles API tank engineering, install, and certification.

Sub-Specs You Need to Know

API gravity is temperature-dependent

API gravity is reported at 60 deg F (15.56 deg C). Field-measured at 80 deg F, the same fluid reads ~1.5 deg API higher. Custody-transfer measurement always converts to 60 deg F via ASTM D1250 / API MPMS Chapter 11.1 tables. For tank-sizing, this matters when summer temperatures push observed-volume higher than nameplate-volume.

Specific gravity is density-ratio, not weight-ratio

Fluids with absorbed gas (live crude, oxygenated water, fizzed beverages) have lower density than the same fluid degassed. Tank gauging on a degassing flow stream needs gas-corrected density measurement. For commodity industrial chemistry, this is rarely an issue; for hydrocarbon process service, always confirm whether the SG quoted is at-condition or after-stabilization.

Density varies more than you think with temperature

Water density at 32 deg F is 0.99987 g/cm3; at 100 deg F is 0.99318 g/cm3. A difference of 0.7%. For most tank-sizing it doesn't matter. For petroleum it absolutely matters: gasoline density swings ~0.7% per 20 deg F, and on a 10,000-gallon receipt that's a 70-gallon discrepancy if not temperature-corrected.

The "1.0 SG = 8.345 lb/gal" shortcut is true for water only

The 8.345 lb/gal figure is the density of water at standard temperature. It is NOT a unit conversion factor that applies to all liquids. For other liquids, multiply SG x 8.345 to get lb/US gallon at the same temperature. For 1.5 SG ferric sulfate: 1.5 x 8.345 = 12.52 lb/gal.

Common Mistakes

Mistake 1: Reading 35 deg API as 1.35 SG

Inverted. API of 35 corresponds to SG 0.850 (lighter than water). API gravity is upside-down vs SG. Any time you see deg API above 10, the fluid is lighter than water; below 10 it's heavier. Always confirm the conversion before assuming.

Mistake 2: Buying a 1.0-SG-rated tank for "water" without considering chlorinated brine fill

Many "water" tanks end up holding sodium hypochlorite, calcium chloride brine, sodium chloride brine, glycol antifreeze, polymer-thickened water. Each adjusts SG up. A 1.0 SG-rated tank holds water but is undersized for 12% NaCl brine (SG 1.09) at full fill, with no margin. Default a tank to 1.5 SG even for nominal water service.

Mistake 3: Ignoring temperature in petroleum custody transfer

The 100-bbl receipt at 88 deg F gauges 100.4 bbl on the ticket if not temperature-corrected. Over a year of receipts, this compounds into pricing disputes. API MPMS Chapter 11 conversion is non-optional.

Mistake 4: Using SG for pump head calculations without unit-converting

Pump TDH math requires consistent units. Density in lb/ft3 or kg/m3, not SG. SG x 62.428 = density in lb/ft3 for water-baseline at 4 deg C; close enough for engineering at ambient temperatures.

Mistake 5: Confusing API gravity with API tank ratings

"API 12F" is a tank construction standard. "30 deg API" is a fluid property. They share the API initialism (American Petroleum Institute) but are independent specifications. Don't conflate.

Field-Buyer Quick Reference

If your supplier or chemistry datasheet quotes density, use SG = density (g/cm3) directly for tank-rating math.
If your supplier quotes API gravity, convert to SG = 141.5 / (deg API + 131.5) and use SG for tank rating.
Verify operating temperature - SG ratings on PE tanks are 73 deg F values; derate for hot service.
For chemistry above 1.0 SG, default to 1.9 SG-rated tank (XLPE for any chemistry above 1.5 SG).
For petroleum service: polyethylene is not the answer; consult Specialty & Metal Fabrication.
Foundation pad load = SG x 8.345 lb/gal x tank capacity in gallons + tank dead weight.
Document chemistry SG, temperature, and volume on the asset tag.

Internal Resources

Source Citations

ASTM D1429 - Standard Test Methods for Specific Gravity of Water and Brine
ASTM D854 - Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer
ASTM D1250 - Standard Guide for Use of the Petroleum Measurement Tables
ASTM D287 - Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method)
ASTM D4052 - Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter
ASTM D1998 - Standard Specification for Polyethylene Upright Storage Tanks
API MPMS (Manual of Petroleum Measurement Standards) Chapter 11.1 - Volume Correction Factors
API MPMS Chapter 3.1A - Standard Practice for the Manual Gauging of Petroleum and Petroleum Products
API 650 - Welded Tanks for Oil Storage
API 12F - Specification for Shop-Welded Tanks for Storage of Production Liquids
40 CFR 264.193 - Containment and Detection of Releases (RCRA secondary containment)
ACI 318 - Building Code Requirements for Structural Concrete
IBC Section 1808 - Foundations