A single molecule of anhydrous ammonia (NH₃) has a molecular weight of 17.03, making it one of the lighter compounds you will encounter in agriculture or industry. But “how much does it weigh” depends heavily on whether you are asking about the gas, the pressurized liquid, or a full nurse tank sitting in a field. A gallon of liquid anhydrous ammonia weighs roughly 5.15 pounds at 60 °F, which is significantly lighter than a gallon of water. That seemingly simple number shifts with temperature and pressure, and it carries real consequences for transportation, storage safety, and how much nitrogen actually reaches your soil.
Molecular Weight and Why It Matters
Anhydrous ammonia is made up of one nitrogen atom bonded to three hydrogen atoms. The NIST Chemistry WebBook lists its molecular weight at 17.0305 grams per mol.1NIST Chemistry WebBook. Ammonia That is remarkably light for a substance with such industrial punch. Water, by comparison, has a molecular weight of about 18, and carbon dioxide comes in near 44. The low molecular weight of ammonia is the reason it behaves the way it does: it boils at a very low temperature, it is lighter than air as a gas, and it packs a high percentage of nitrogen per pound. Roughly 82% of anhydrous ammonia’s weight is nitrogen, which is why farmers have relied on it as one of the most nitrogen-dense fertilizers available.
Weight as a Gas Versus Weight as a Liquid
Anhydrous ammonia exists as a gas at normal atmospheric temperatures and pressures. At standard conditions, ammonia gas has a density of about 0.73 kilograms per cubic meter. Air, by comparison, sits around 1.22 kilograms per cubic meter under the same conditions. So ammonia gas is roughly 60% as heavy as the surrounding air, which is why it tends to rise and disperse when released outdoors.
Under pressure or at very cold temperatures, ammonia becomes a liquid, and that is the form it takes inside the white nurse tanks you see being towed behind tractors. Liquid anhydrous ammonia is dramatically denser than the gas. At its atmospheric boiling point of about −28 °F (−33 °C), the liquid weighs around 5.7 pounds per gallon. At a more typical handling temperature of 60 °F, where the ammonia must be kept under roughly 125 pounds per square inch of pressure to stay liquid, density drops to about 5.15 pounds per gallon. At 100 °F, a gallon weighs closer to 4.8 pounds. This temperature sensitivity is not a trivial detail. It dictates how full a tank can safely be loaded and how much product you are actually hauling to the field.
How Temperature Changes the Numbers
Liquid ammonia expands as it warms. A tank loaded to 100% capacity on a cool morning can develop dangerously high internal pressures by afternoon if the temperature climbs. That is why ammonia tanks are typically filled to no more than 85% of their water capacity. The fill limit accounts for the fact that if the liquid warms and expands, it needs room to do so without overpressurizing the vessel.
Here is what that looks like across a range of common temperatures:
- −28 °F: About 5.7 lb per gallon, the liquid at atmospheric boiling point
- 0 °F: About 5.5 lb per gallon
- 60 °F: About 5.15 lb per gallon, a common reference point
- 80 °F: About 5.0 lb per gallon
- 100 °F: About 4.8 lb per gallon
The weight per gallon drops by nearly a pound across that temperature range. If you are calculating how many pounds of nitrogen you are applying per acre, you need to know the temperature of the ammonia in your tank, not just how many gallons you loaded. Two farmers filling the same volume of ammonia on different days could be hauling meaningfully different amounts of nitrogen.
Common Tank Sizes and Their Payloads
Most practical questions about anhydrous ammonia weight come down to tanks. The nurse tanks used for direct field application and the larger storage vessels at dealerships hold very different amounts, and understanding the weight in each matters for both transport regulations and equipment safety.
A standard 1,000-gallon nurse tank, filled to the 85% mark with liquid ammonia at 60 °F, holds approximately 4,378 pounds of ammonia. The tank itself typically weighs somewhere around 2,200 to 2,600 pounds when empty, so the loaded assembly is in the neighborhood of 6,500 to 7,000 pounds rolling down the road. That is a lot of weight for a piece of equipment often pulled by a farm tractor on public roads, which is one reason state regulations around transport are strict.
Larger 1,450-gallon nurse tanks are also common, and at the same 85% fill and 60 °F, they carry roughly 6,350 pounds of ammonia. Bulk storage tanks at agricultural dealerships can range from 6,000 to 30,000 gallons or more. A 12,000-gallon storage tank at 85% fill holds in the ballpark of 52,500 pounds of ammonia. Rail tank cars used to ship ammonia from production facilities to regional hubs are much larger still, with a typical car carrying around 150,000 to 180,000 pounds of product.
Lighter Than Air, Until It Is Not
Because ammonia gas is lighter than air, there is a common assumption that a release will simply float up and away. Under some conditions, that is exactly what happens. Pure ammonia vapor, even at its cold boiling-point temperature, is less dense than ambient air. But researchers studying atmospheric dispersion have found that a cloud of ammonia mixing with air can actually become denser than the surrounding atmosphere under certain circumstances.2Journal of Chemical Technology and Biotechnology. The density of an ammonia cloud in the early stages of atmospheric dispersion
The physics behind this are counterintuitive. When cold ammonia vapor mixes with warmer, humid air, it cools that air down sharply. Cold air cannot hold as much moisture, so water begins condensing into tiny droplets, forming fog. Ammonia also dissolves readily into those water droplets, creating an ammonia-water fog. The resulting cloud contains liquid droplets that add weight, and the mixture can become heavier than the air around it.3Physics of Fluids. Influence of ammonia-water fog formation on ammonia dispersion from a liquid spill This means a large release of liquid ammonia, such as from a ruptured tank, can produce a ground-hugging toxic cloud rather than a plume that rises and disperses. Emergency responders train for this scenario specifically because the weight behavior of ammonia during a spill does not match what the pure-gas density would predict.
Condensing water droplets do release some latent heat, which warms the mixture and pushes it back toward buoyancy, so the physics involves competing effects. In humid conditions with a large spill, the droplet weight wins out and the cloud stays low. In dry conditions with a small leak, the vapor rises as expected. The weight of the ammonia cloud, in other words, depends on the weather as much as the chemistry.
How Weight Translates to Nitrogen on the Field
For farmers, the weight of anhydrous ammonia is directly tied to how much nitrogen ends up in the soil. Since ammonia is about 82% nitrogen by weight, every 100 pounds of anhydrous ammonia delivers roughly 82 pounds of actual nitrogen. That makes it the most nitrogen-dense fertilizer commonly available. Urea, the next most concentrated solid source, is about 46% nitrogen. Ammonium nitrate sits around 34%. So pound for pound, anhydrous ammonia delivers roughly twice the nitrogen of urea and more than twice that of ammonium nitrate.
But weight applied is not the same as weight retained. When anhydrous ammonia is knifed into the soil, the cold liquid instantly vaporizes and reacts with soil moisture to form ammonium ions that bind to soil particles. How well the soil holds onto that nitrogen depends heavily on soil pH. Research on ammonia retention has shown that in acidic soils with a pH below 6, roughly 92% of the injected nitrogen is still present 24 hours after application. In alkaline soils with a pH above 7.5, retention drops to around 64%.4Canadian Journal of Soil Science. Retention and nitrification of injected anhydrous NH3 as affected by soil pH That roughly 30-percentage-point gap means the effective weight of nitrogen your crop sees can vary substantially depending on where you farm. A thousand-gallon tank of ammonia delivers the same weight of nitrogen into every field, but how much stays put is a different question.
Injection depth matters for the same reason. Shallow injection in dry soil allows more ammonia gas to escape back to the surface before it can react with moisture. Deeper injection, typically six to eight inches, gives the ammonia more soil volume to react with and reduces losses. The weight on the application rate chart is the weight going in, not necessarily the weight your corn will be able to use.
Weight and Pressure in Storage
Anhydrous ammonia’s critical temperature is about 270 °F (roughly 405 K), and its critical pressure is around 1,640 psi (113 bar).1NIST Chemistry WebBook. Ammonia Below these values, ammonia can exist as a distinct liquid under its own vapor pressure. Above them, it becomes a supercritical fluid where the boundary between liquid and gas disappears. Practical storage happens well below these extremes, but understanding the relationship between pressure and temperature helps explain why ammonia tanks are engineered the way they are.
At 60 °F, the vapor pressure of ammonia is around 108 psi. At 100 °F, it climbs to about 198 psi. That is why nurse tanks and storage vessels are rated to handle at least 250 psi, with safety valves set to relieve pressure before the vessel walls are stressed beyond design limits. The weight of ammonia inside is what creates this pressure. More liquid in the tank means less room for the vapor above it, and if the liquid expands enough to eliminate the vapor space entirely, pressure can spike rapidly. Overfilling is one of the most dangerous mistakes in anhydrous ammonia handling, and it comes down to not accounting for how the weight-to-volume relationship shifts with temperature.
Transport Regulations and Weight Limits
Moving anhydrous ammonia on public roads means dealing with hazardous materials regulations. In the United States, the Department of Transportation classifies anhydrous ammonia as a Division 2.2 nonflammable compressed gas with an additional inhalation hazard designation. Weight limits for transport depend on the vehicle type and local road restrictions, but a loaded nurse tank on a trailer typically falls within the weight range that requires placarding and compliance with hazmat transport rules.
A practical issue that comes up frequently: farmers sometimes underestimate the total weight they are pulling. A 1,000-gallon nurse tank loaded to 85% with ammonia at moderate temperatures weighs close to 7,000 pounds. Add the trailer, and you can easily exceed 8,000 pounds. Hitch ratings, tire load capacities, and brake requirements all need to match that weight. The ammonia itself is heavy enough that a fully loaded nurse tank rivals the weight of a small car, and the center of gravity shifts as the liquid sloshes during transport. Bridge weight limits in rural areas can also be a factor, especially when moving multiple tanks to a distant field.
Ammonia as a Fuel and Industrial Feedstock
Outside of agriculture, ammonia’s weight characteristics are drawing fresh attention from the energy sector. Because ammonia can be liquefied at relatively modest pressures and stores about 17.6% hydrogen by weight, it is being evaluated as a carrier for hydrogen fuel. A liter of liquid ammonia holds more hydrogen atoms per volume than a liter of liquid hydrogen itself, largely because liquid hydrogen is extremely light (only about 0.57 pounds per gallon) and must be kept at cryogenic temperatures far colder than ammonia requires.
For shipping and heavy transport, the weight-to-energy tradeoff matters enormously. Ammonia’s energy density by weight is lower than conventional fuels like diesel, meaning you need to carry more pounds of ammonia to travel the same distance. But ammonia’s advantage is that it can be made from renewable electricity, water, and air, offering a potential zero-carbon fuel cycle. The infrastructure already exists to store and transport millions of tons of ammonia annually for the fertilizer industry, so the weight and handling characteristics are well understood. Whether ammonia’s relatively low energy per pound can be offset by its easier storage compared to pure hydrogen is one of the central engineering questions in the emerging ammonia-fuel space.
Ammonia Versus Water and Other Common Liquids
Putting ammonia’s weight in context helps make the numbers feel real. At 60 °F, a gallon of liquid anhydrous ammonia weighs about 5.15 pounds. A gallon of water weighs about 8.34 pounds. A gallon of gasoline weighs around 6.1 pounds. So liquid ammonia is roughly 62% as heavy as water and about 84% as heavy as gasoline at similar temperatures.
This lower density is part of why ammonia tanks look oversized for the weight they carry. A 1,000-gallon ammonia tank delivers less than 4,400 pounds of product at typical fill levels, while a 1,000-gallon water tank would hold over 7,000 pounds. If you are used to estimating loads based on water weight, ammonia will always feel lighter than you expect. That can cut both ways: lighter loads mean less stress on equipment per gallon, but the chemical’s toxicity and high vapor pressure make it far more hazardous to handle than its modest weight per gallon might suggest.
The difference also matters inside pipelines and transfer systems. Because ammonia is less dense, pumps designed for water will move ammonia at a higher volumetric flow rate for the same energy input, but the mass flow rate will be lower. Flowmeters calibrated for water will overestimate the weight of ammonia passing through them unless corrected for density. Facilities that handle both ammonia and water-based solutions need instrumentation set up for each fluid’s specific gravity.