Field Area Calculator: Plan Seed and Fertilizer by the Acre

Published November 7, 2026

A field area calculator gives you the one number every input order starts from: how many hectares or acres you actually plant. Measure the field boundary on a satellite map or by walking it with GPS, subtract anything inside it you don't crop, then multiply that area by the rate on the seed bag, fertilizer plan or spray label. Getting the area right matters more than most people expect: a field guessed at 5 ha that is really 4.4 ha means 13% too much of everything you buy.

This guide shows how to measure the field, what to leave out, and how to turn the area into bags and tank loads, with a full worked example.

Why the area is worth measuring properly

Almost every rate in farming is "per hectare" or "per acre": seed in kg/ha or seeds per acre, fertilizer in kg/ha or lb/acre, spray volume in L/ha, contract work priced by the acre. Multiply a rate by the wrong area and the error carries straight through:

• Too high an area: you buy and spread more than the field needs. Extra fertilizer costs money and can end up in ditches and streams.
• Too low an area: you run out halfway across the field, or stretch the last bags thin and get an uneven crop.

Old figures are the usual culprit. The area on a lease, a tax record or "what Dad always said" may include the yard, a pond or a strip that was sold years ago. If you haven't measured it yourself, it's worth an hour.

Step 1: Measure the field boundary

There are two practical ways to do it from a phone:

1. Tap the corners on a satellite map. Zoom in close on each corner so the point sits on the real field edge, not on a tree shadow or the middle of a hedge. This is quick and works for most open fields. Measuring land by satellite or GPS covers the technique.
2. Walk or drive the edge with GPS. Better where trees hide the edge, or where two fields look the same from above. Walk at a steady pace, keep to the same side of the fence or ditch, and close the loop where you started. Measuring land by walking has the details.

Under open sky, phone GPS is typically accurate to about 3–5 m, and a corner tapped on a clear image can still be off by a metre or two. On a field of a few hectares that adds up to roughly ±3–5% of the area, which is precise enough to plan inputs, and far better than a guess.

Step 2: Take out what you don't plant

The boundary is not the cropped area. Before you multiply anything, measure and subtract:

• ponds, wet patches and rocky outcrops you drive around,
• the farmyard, sheds, a well or pump house inside the boundary,
• permanent tracks and wide grass margins,
• any part you're leaving fallow or planting with something else.

Measure each one as its own small shape and subtract it from the field total. Headlands that you do plant stay in.

Step 3: Turn the area into amounts

The maths is a single multiplication per input:

amount = cropped area × rate

Then divide by the bag or tank size and round up, because you can't buy 0.3 of a bag. If your rates are per acre and your area is in hectares (or the other way round), convert first: 1 ha = 2.471 acres, and 1 acre = 0.405 ha. A rate of 160 kg/ha is about 143 lb/acre.

Worked example: a wheat field

The rates below are examples only. Use the ones from your seed supplier, agronomist or product label.

The field. Tapped on the satellite map, the boundary comes to 4.62 ha (11.42 acres). Inside it there is a pond of 0.08 ha and a farmyard of 0.12 ha, measured separately.

Cropped area: 4.62 − 0.08 − 0.12 = 4.42 ha (10.92 acres).

The difference it makes. If the field had been planned as "about 5 ha", the seed order would have been 800 kg, 93 kg more than needed, and the fertilizer 1,250 kg instead of 1,105 kg. That is 13% extra on every line of the bill.

How sure is 4.62 ha? The field is roughly 260 × 178 m, so its perimeter is about 876 m. If each edge could be off by 2 m, the area could move by about 876 × 2 = 1,752 m², or 0.18 ha, around 4%. That's the size of the uncertainty, and it's still far smaller than the error in the guess.

Measuring fields with Field Maps with Area Measure

Field Maps with Area Measure does both measuring methods on iPhone and iPad (iOS 17 or later): tap points on the satellite map, with a magnifier loupe for placing corners precisely, or walk, drive or fly the boundary and let the app record the path and close the shape. It shows area and perimeter in hectares, acres, m² and more, and you can save each field and each excluded area with its own name, so next season the numbers are already there. GPS tracking works offline, which helps on fields with no signal. Pro adds splitting a field into sections and exports such as Shapefile, KML and GeoJSON for other farm or GIS software. There's no Android version yet.

Common mistakes

• Using the boundary instead of the cropped area. The pond and the yard are inside the line but don't get seed.
• Mixing units. A per-acre rate multiplied by hectares gives 2.47 times too little; the reverse gives 2.47 times too much.
• Rounding the area before multiplying. Round the final order, not the field.
• Forgetting slope. Phone apps, like maps and most surveys, give horizontal area. On a steady 15% slope the ground surface is only about 1.1% larger, so for input planning you can usually ignore it.
• Measuring once, years ago. Fields change: a track gets widened, a corner gets fenced off. Re-measure when the shape changes.

FAQ

How do I calculate the area of a field?

For a rectangle, multiply length by width. For any other shape, measure the boundary on a satellite map or walk it with GPS, and the app calculates the area for you. For hand calculation of odd shapes, see how to calculate the area of irregular land.

How much fertilizer do I need per acre?

It depends on the crop, the soil and the product, so take the rate from a soil test, your agronomist or the label. Once you have it, multiply the rate by the planted acres and divide by the bag size.

Is a GPS field area calculator accurate enough for ordering inputs?

Yes. On a field of a few hectares, a careful phone measurement is usually within a few percent, which is much closer than a remembered or rounded figure. For legal boundaries you still need a licensed surveyor.

Should I include headlands in the area?

Include them if you plant them. Leave out only the parts that never get seed or fertilizer, such as ponds, yards and permanent tracks.