What this site is
Field & Farm Calc is a set of free calculators for the practical arithmetic of rural property: how many bales it takes to get a herd to spring, how many acre-inches of forage a paddock is actually carrying, how much seed a drill is really putting down per acre, how many gallons are in a pond, what a cord of red oak is worth against propane at today's price, what pasture rents for in your state, and roughly when the rut peaks where you hunt.
All of it runs in your browser. Nothing you type is sent anywhere, there is no account to create, and there is no app to install. Most of these pages get opened on a phone, outdoors, with one hand, by somebody standing next to the thing they are trying to measure. That constraint drove most of the design decisions here.
The site exists because this math is not difficult, but it is scattered. The formula for grazing days lives in a University of Kentucky PDF. The conversion factor for spreader calibration lives in a Penn State fact sheet. The Btu content of a cord of white oak lives in a Utah State forestry table. Cash rent lives in a USDA database that most people have never heard of. Each of those is public and each is authoritative, and almost nobody standing in a field has all four open at once. The useful thing this site does is collect them, do the fiddly part, and show its work.
Who builds it
Field & Farm Calc is built and maintained by one person — an independent developer, not an extension agent, not an agronomist, not a fisheries biologist, and not a company. Nobody pays to appear here. No calculator recommends a product, no page is sponsored, and none of the market data is supplied by anyone with a stake in the price. The site is funded by advertising, which is disclosed in the privacy policy.
That matters for how you should read the site, so it is worth being blunt about it. The value here is not a professional's judgment about your ground. There is no professional judgment on this site at all. The value is that every constant and every formula is traceable to a public source that you can open and check yourself, and that the arithmetic in between is tested. Where a source publishes a range instead of a number — and for most of this subject matter it does — the range is shown as a range. Where the published research does not cover a state or a species, the page says so rather than filling the gap with something plausible.
Where the numbers come from
The rule is: published extension-service, university, or federal-agency sources, named on the page where the number is used, with the year. Not blog posts, not forum wisdom, not an average of whatever the search results said.
- Spreader calibration — Penn State Extension's manure spreader calibration procedure: catch a pass on a tarp of known area, weigh it, divide pounds by square feet, and multiply by 21.8 to reach tons per acre. That factor is not arbitrary; it is 43,560 square feet per acre divided by 2,000 pounds per ton. The page uses Penn State's method as published, including the three-pass layout that captures the overlap, and the instruction to drive at the speed you normally spread at rather than a careful calibration crawl.
- Drill and seeder calibration — Penn State Extension's grain and seed drill calibration fact sheet. Working width is opener spacing times opener count; acres covered is width times distance divided by 43,560; rate is seed weight divided by acres. Penn State's own worked example — 2.3 pounds collected over 0.046 acre coming out to 50 pounds per acre — is one of this site's automated tests, so if the calculator ever stopped agreeing with the published example, the build would fail before it shipped. The same fact sheet is the source for the per-opener check and the ten percent deviation threshold that flags a tube needing maintenance.
- Grazing, stocking rate, and paddocks — the University of Kentucky forage program's grazing stick publication supplies dry matter per acre-inch by species and stand density, utilization percentages by rotation speed, and intake as a percentage of body weight by class of livestock. Its published equation keeps utilization and intake as two separate terms, which is the detail that most rules of thumb quietly get wrong by folding them together and double-counting. Paddock count follows West Virginia University Extension: rest days divided by grazing days per paddock, plus one, rounded up.
- Firewood and heating cost — Utah State University Forestry Extension's wood heating tables for heat content per cord by species, in million Btu per cord of dry wood, with no appliance efficiency baked in. That last part matters. Some widely circulated tables publish "available" Btu with a stove efficiency already applied; running those through an efficiency multiplier a second time understates a cord by half. This site applies efficiency exactly once, deliberately, and uses the un-baked values as its starting point. Competing fuels use standard energy constants: 91,452 Btu per gallon of propane, 100,000 Btu per therm of natural gas, and 3,412 Btu per kilowatt-hour.
- Hay prices — USDA Agricultural Marketing Service hay market reports, with the specific report name, date, and link retained for each row rather than blended into a single national figure. Where a state's number comes from a university extension report instead of AMS, the page says which.
- Cash rent — USDA National Agricultural Statistics Service cash rent survey data, pulled directly from the QuickStats database for all reporting states across non-irrigated cropland, irrigated cropland, and pasture. A sample of those values was cross-checked by hand against NASS's own published releases before the page went up. County-level detail is linked to NASS rather than republished, because county estimates are not published everywhere and pretending otherwise would be worse than a gap.
- Hunting lease rates — published state-level lease surveys, each shown as a range with the survey year attached: University of Georgia, University of Missouri Extension, Kansas State, LSU AgCenter, Penn State, and Mississippi State. These surveys were run years apart under different methods, so they are never averaged together into one national number. A 2001 Kansas figure and a 2023 Missouri figure are two different facts, not two samples of the same fact.
- Rut timing — state wildlife agency breeding-date studies, most of them based on fetal measurement from harvested does, which is the closest thing to hard evidence this subject has. Each state's row carries its own agency source. States where the agency figure is a published estimate rather than a fetal study are labeled as estimates, and the states with no usable published source at all are listed by name as having none.
- Ponds and fish — standard volumetric and length-weight relationships, with the assumptions about pond shape and average depth stated on the page, because those assumptions are where the error lives, not in the arithmetic.
If a number on this site appears without a source next to it, that is a defect. Report it and it gets a source or it gets removed.
How the math is checked
Every calculator has a matching test that re-implements its formula independently and checks the result. Where the source authority publishes a worked example, that example becomes a test — the drill calibration example above is the clearest case, and it means the page is checked against Penn State's answer rather than against its own. Where no worked example exists, the tests check internal consistency and the boundaries instead: that unit conversions round-trip, that a doubled herd halves the grazing days, that a species with no published dry matter figure returns nothing rather than a silent default, that efficiency is applied exactly once.
The suite also tests the words, not just the numbers. It checks that source citations are present on the pages that carry rates, and that the pages which are meant to be free of advertising stay that way. Nothing deploys unless the whole suite passes — currently sixty-six assertions across eighteen pages.
This does not make the site infallible. A test can only check the formula it was given, and a formula can be faithfully implemented and still be the wrong formula for your ground. What testing does buy is that a typo in a constant, a broken conversion, or a page that silently lost its sourcing gets caught before anyone reads it.
What these calculators will not tell you
They will not tell you what your land, your herd, your pond, or your woodlot is going to do.
This is not a disclaimer bolted on at the end; it is the honest limit of the method. Every figure on this site is a published average or a published range, and rural averages have enormous spread. Two neighboring pastures can differ by a factor of two in dry matter per acre-inch depending on stand density, fertility, and how they were grazed last year. A cord of oak that was split last month and a cord split two years ago are not the same fuel — green firewood can be half water by weight, and burning it wastes energy boiling that water off. A hay requirement calculated from body weight assumes a feeding waste percentage that your bale rings may or may not match. A rut peak from a fetal study describes a statewide midpoint, not the week the deer move on your property.
So use these numbers the way you would use a first estimate: to size a decision, to check whether something is off by ten percent or by a factor of three, to walk into a conversation with a number in hand. For anything where being wrong is expensive or irreversible — a stocking decision, a pond treatment, a livestock health question, a lease you are about to sign — talk to your county extension office, your veterinarian, or your state fisheries biologist. They can see your ground. This site cannot.
How often it's reviewed
Pages carrying prices, rates, or survey data are re-checked against their sources at least twice a year, and immediately when a source publishes new data — a new USDA cash rent release, a new AMS hay report cycle, a revised extension publication. Pages built on physical constants and published formulas change far less often and are re-checked annually. The current review pass is July 2026.
Corrections
If a number here is wrong, a source has been superseded, or a page reads in a way that would lead somebody to a bad decision, say so: [email protected]. Corrections take priority over new features. Include the page and, if you have it, the publication that contradicts what is on the page — that turns a fix from a research project into ten minutes.
Requests for calculators that do not exist yet are welcome too, and several pages on this site started as one.
Privacy, briefly
Whatever you type into these calculators stays on your device. They are plain JavaScript running in your browser; there is no server that receives your acreage, your herd size, or your lease terms, because there is no server involved beyond delivering the page itself. Full details, including how the advertising works, are in the privacy policy.