Propane Tank Refill vs Exchange Cost Calculator

The hidden math behind propane tank swaps

Backyard grilling, patio heaters and portable generators all run on the humble 20-pound cylinder. When the flame starts to sputter you face a small but surprisingly lopsided choice: drive to a refill station and pay by the gallon, or walk up to a swap rack and pay a flat fee for someone else's refurbished tank. The swap takes ninety seconds and the refill takes a trip across town, but the money moves in the opposite direction. A refill meters exactly the gallons you are missing. A swap charges the same whether you hand over a dry cylinder or one that is three-quarters full, and every gallon still inside that cylinder becomes a gift to the rack operator. This calculator prices both sides of that trade, including the two figures most comparisons quietly skip: the fuel you forfeit at the swap, and the fact that an exchange cylinder is usually not filled to the same level a refill station would fill yours.

Introduction: why an exchange cylinder holds less than a refill

Propane cylinders are never filled to the brim, because liquid propane expands about 1.5 percent for every 10 degrees Fahrenheit of temperature rise and the vapour space above the liquid is what absorbs that expansion. Federal rules cap the fill: OSHA's LP-Gas table sets a maximum permitted filling density of 42 percent by weight for propane in containers of this size, and NFPA 58 requires an overfill prevention device on every 4-to-40-pound cylinder to enforce it. A nominal 20-pound grill cylinder has a water capacity around 47.6 pounds, so 42 percent of that is roughly 20 pounds of propane. At about 4.24 pounds per gallon at 60 degrees Fahrenheit, that is 4.7 gallons — the number this page uses as the default capacity.

Exchange racks are a different animal. Many of the big national programmes fill their cylinders to 15 pounds rather than 20, which is roughly 3.5 gallons. The tank looks identical, the price tag says one flat number, and nothing on the cage tells you that you are buying about three-quarters of the propane a refill station would have put in your own cylinder. That is why this calculator asks for the exchange fill separately: comparing a $17 swap against a $17 refill is meaningless if one of them delivers 3.5 gallons and the other delivers 4.7.

How to use this refill versus exchange comparison

1. Enter your tank capacity in gallons. A standard 20-pound grill cylinder holds about 4.7 gallons at the legal fill limit, which is the default; use roughly 7.1 gallons for a 30-pound tank and 9.4 for a 40-pound tank.
2. Enter the refill price per gallon posted at your local refill station, hardware store or propane dealer.
3. Enter the flat exchange price per tank charged at the swap cage outside the grocery or gas station.
4. Enter the gallons an exchange cylinder contains. Leave it at 3.5 for the common 15-pound fill, or raise it to match your capacity if your local programme fills to 20 pounds.
5. Estimate the gallons remaining in the tank you would be handing over — from the float gauge, from the tank's heft, or by weighing it and subtracting the tare weight stamped on the collar.
6. Optionally add the round-trip cost of the refill run: fuel, tolls, or a dollar value you put on the twenty minutes. Leave it at zero to compare sticker prices only.

Press Compare options and the result panel reports both readings — which option writes the smaller cheque today, and which one delivers propane at the lower cost per usable gallon. Those two answers disagree more often than you would expect, and the panel says so explicitly when they do.

Formulas for refill cost, exchange cost and the break-even leftover

The refill bill is the gap between capacity and what is already in the tank, priced by the gallon, plus whatever the trip costs you:

The exchange bill is a constant. It does not care about L at all, which is the whole source of the asymmetry:

Here V is tank capacity in gallons, L the leftover gallons in the tank you are handing over, P_r the refill price per gallon, P_e the flat exchange fee, and T the round-trip cost of the refill run. Setting the two costs equal gives the classic same-tank break-even leftover:

Carry more than L_be gallons back to the station and the refill is the smaller cheque; carry less and the flat fee is. That threshold answers the question most people actually ask, but it silently assumes both options leave you with the same amount of propane. They usually do not.

Effective price per usable gallon: the number the sticker hides

A fair comparison divides what you pay by what you actually gain. A refill takes you from L gallons to V gallons, so it nets you V − L gallons:

An exchange takes you from L gallons to the rack's fill level E, so it nets you only E − L gallons for the same flat fee:

When L approaches E that denominator collapses and the effective price per gallon runs away to infinity. When L exceeds E the swap hands you less propane than you walked in with, and there is no price per gallon to quote at all — you paid money to lose fuel. The calculator flags that case in plain language instead of printing a meaningless number.

Worked example: a Fourth of July fill-up

Your local refill station is charging $4.25 a gallon over a busy holiday weekend, and the swap cage outside the grocery store lists tanks at a flat $17.00 with the usual 15-pound (3.5 gallon) fill. Your grill's 20-pound cylinder holds 4.7 gallons, and hefting it after the last cookout you put the leftover at about 2.0 gallons. Take the refill run as free for now.

The refill buys the missing 2.7 gallons: 2.7 × $4.25 = $11.48, and you keep the 2.0 gallons you already own, ending with a full 4.7-gallon tank. The swap costs $17.00, hands you a 3.5-gallon cylinder and quietly takes your 2.0 gallons, worth $8.50 at the same price. Net gallons gained: 2.7 for the refill against 1.5 for the swap. On cost per usable gallon that is $4.25 versus $11.33 — the refill is not slightly better here, it is close to three times better.

The same-tank break-even sits at 4.7 − ($17.00 ÷ $4.25) = 0.70 gallons, about 15 percent of capacity, so anything above roughly three-quarters of a gallon makes the refill the smaller cheque as well. Arrive bone-dry and the picture flips on outlay alone: a full refill costs 4.7 × $4.25 = $19.98 against $17.00 for the swap. But look at the per-gallon column before you celebrate — that $19.98 buys 4.7 gallons at $4.25 each, while the $17.00 buys only 3.5 gallons at $4.86 each. The swap saves you $2.98 today and sells you propane at a 14 percent premium.

Scenario table: how the two costs cross as the tank empties

Holding the $4.25 refill price, the $17.00 flat fee and the 3.5-gallon exchange fill steady, here is the whole curve. Read the fourth and fifth columns together: the outlay column answers what do I hand over at the till today, and the per-gallon column answers what am I paying for propane. They only agree once the tank is nearly empty.

The bottom two rows are the ones worth remembering. Once your leftover reaches the exchange fill level, a swap takes propane away from you and charges you for the privilege. Enter your own numbers below and the calculator rebuilds this table for your prices.

Real-world factors beyond the sticker price

Price and leftover fuel dominate the arithmetic, but they are not the whole decision. Refill stations keep business hours and sometimes close on holidays — exactly the weekends when you need propane most — while swap cages sit outside 24-hour convenience stores. A refill attendant inspects your cylinder and will refuse a tank whose requalification date has passed, which under the US Department of Transportation rules is 12 years from manufacture, with 5- or 12-year extensions after inspection. That is a genuine benefit disguised as an inconvenience: an expired cylinder is a cylinder nobody has looked at in over a decade. Swap racks accept expired, rusted and dented tanks without comment and take the requalification burden off your hands, which for a cylinder that is already at the end of its life is worth real money.

Supply matters too. Racks run out on holiday weekends, refill queues grow, and a household that keeps a second cylinder as a spare never has to make the decision under pressure at all. The cost of that spare tank is a one-time purchase amortised across every cookout you do not have to abandon.

Why accurate leftover estimates matter

Everything above hinges on L, and L is the one input nobody measures properly. A float gauge on the collar is a rough guide at best and reads differently at different temperatures. The reliable method is a bathroom scale: weigh the cylinder, find the tare weight stamped on the collar next to the letters TW (typically 17 to 18 pounds for a 20-pound tank), subtract it, and divide the remaining pounds by 4.24 to get gallons. The warm-water trick works when you have no scale — pour warm water down the side and run your hand along the steel, and the point where it turns cool is the top of the liquid.

Errors here are not symmetric. Overestimating your leftover makes the refill look cheaper than it is and, if you were leaning toward a swap, understates how much fuel you are about to throw away. Underestimating it exaggerates the refill saving. Because the exchange side of the comparison is completely flat, every gallon of error moves the refill cost by exactly one gallon's worth of money — so a half-gallon guessing error at $4.25 a gallon is a $2.13 error in the comparison, which is roughly the size of the whole decision when the tank is nearly empty.

Convenience versus cost: pricing the drive

The optional trip-cost field exists so you can settle the convenience argument with a number rather than a shrug. If the refill station is a 20-minute round trip and you value your time at $15 an hour, the trip is worth $5.00; add the fuel and you are near $6.00. Feed that into the calculator and the break-even leftover moves from 0.70 gallons up to 4.7 − ($17.00 − $6.00) ÷ $4.25 = 2.11 gallons — suddenly you need a tank that is nearly half full before the drive pays for itself on outlay. Note what does not change: the cost per usable gallon still favours the refill in this example, because $6.00 spread over 2.7 gallons is $2.22 a gallon on top of $4.25, which is $6.47 against the swap's $11.33. Convenience is a real cost, but it is a cost you pay once per trip, while the per-gallon premium is a cost you pay on every gallon you burn.

Limitations and assumptions of this comparison

A few things push the real cost away from the tidy numbers above. Refill pumps meter by volume, and propane expands and contracts with temperature, so a gallon pumped on a cold morning packs a little more energy than one pumped in the afternoon heat; industry billing standardises on 60 degrees Fahrenheit, but a small hand-filled cylinder is not temperature-compensated. Exchange cylinders are supposed to hold a fixed amount, yet the actual fill can run over or under, and programmes have changed their fill levels over the years without changing the cage. Prices swing hard through grilling season and after supply disruptions, so a comparison you ran in June may not hold in December.

The model also treats the propane already in your tank as fuel you simply forfeit at the swap rack. Technically it can be decanted into another cylinder, but that requires proper equipment and training and is not a driveway job. The trip-cost field is a flat per-trip figure and does not model the case where the refill station is on a route you were driving anyway, in which case the honest value is close to zero. Finally, the numbers here assume a healthy, in-date cylinder; if yours is due for requalification, the swap is buying you a service the refill cannot, and the price comparison stops being the whole story.

Environmental impact of refilling versus swapping

Refilling keeps one cylinder in service for its full life. The exchange model moves steel constantly: empty cylinders are collected, trucked to a refurbishing plant, cleaned, requalified, repainted, refilled and trucked back out. Those miles and that reprocessing are real, and so is the propane vented or burned off during cylinder purging. Against that, the exchange system does pull genuinely unsafe cylinders out of circulation and recycles the steel at end of life, which a homeowner refilling a corroding tank may never get around to. If sustainability is part of your decision, the honest summary is that refilling a sound cylinder is the lighter-footprint choice, and swapping is the right call for a cylinder that has reached the end of its safe life.

Planning for peak grilling demand

Propane retail prices are seasonal and regional; the US Energy Information Administration publishes weekly residential propane prices that make the winter heating peak obvious in the data. For grillers the squeeze is different — summer holiday weekends empty the swap cages and lengthen the refill queues rather than moving the price much. The practical response is to watch the gauge and act in the boring middle of the week, when the station is empty and you are not making the decision with guests already in the garden. Some hardware chains run refill coupons that knock a dollar or two off the per-gallon price, which shifts the break-even leftover meaningfully; the calculator will show you by how much if you enter the discounted price.

The bottom line for grillers

Across one summer of cookouts the difference between a refill habit and a swap habit is usually somewhere between twenty and sixty dollars, and it compounds if you are also running a patio heater through the shoulder season. The rule of thumb is short enough to carry in your head: the fuller the tank you hand over, the worse the swap, and a swap never delivers a full cylinder in the first place. Refill when the station is open and the tank has anything meaningful left in it. Swap when you are bone dry, out of time, or holding a cylinder whose requalification date has run out. Then do the boring safety things regardless — check the valve and hose connection with soapy water, keep the cylinder upright, and store it outdoors and away from ignition sources.

Sources and standards behind these numbers

The physical constants used throughout this page come from published safety codes and federal energy data rather than retailer marketing:

• OSHA 29 CFR 1910.110, Storage and handling of liquefied petroleum gases — Table H-27 sets the maximum permitted filling density at 42 percent by weight for propane with a specific gravity of 0.504 to 0.510 in containers up to 1,200 gallons water capacity. This is why a 47.6-pound water-capacity cylinder is a "20-pound" tank.
• NFPA 58, Liquefied Petroleum Gas Code — requires overfill prevention devices on cylinders of 4 to 40 pounds propane capacity and governs cylinder filling, storage and transport.
• US DOT PHMSA cylinder and OPD guidance — cylinder approval, overfill prevention devices and the requalification interval stamped on the collar.
• US Energy Information Administration, British thermal units — propane heat content of about 91,452 Btu per gallon, the figure behind the energy-per-gallon comparisons above.
• Propane Education & Research Council safety guidance — consumer handling, cylinder inspection and the 4.24 pounds per gallon liquid density at 60 °F used for weight-to-volume conversion.

Frequently asked propane refill questions