What this Tidal Range Calculator does
The Tidal Range Calculator finds the vertical difference between a high-water level and a low-water level. Enter both measurements in metres, using the same chart datum, and the calculator subtracts the low-water value from the high-water value. Its result is the tidal range in metres for the two levels you supplied.
For example, if a tide reaches 2.50 m at high water and falls to 0.50 m at low water, the range between those readings is 2.00 m. The figure describes the vertical span from one level to the other. It does not tell you when high or low water will occur, how quickly water will move, or what the tide will be at another time or place.
This distinction matters in marine and navigation contexts. A range is a measurement derived from two water levels. A tide prediction needs location-specific observations or models and a time reference. This calculator performs only the arithmetic difference, so it is useful when you already have suitable level values and want a consistent result.
How to use the Tidal Range Calculator
Enter the high-water and low-water levels in metres. Use values that refer to the same location, measurement system and chart datum. The calculator reports the difference to two decimal places.
- Find the high-water level from the tide table, survey, gauge record or other source relevant to your task.
- Find the low-water level associated with the tide or observation period you are examining.
- Check the units and datum before entering either value. Both should use metres and the same vertical reference.
- Read the tidal range shown in metres. It is the high-water level minus the low-water level.
Suppose your source gives a high-water level of 3.18 m and a low-water level of 0.74 m, both relative to the same datum. The calculation is 3.18 m − 0.74 m = 2.44 m. The calculator returns a tidal range of 2.44 m.
The inputs accept levels from 0 to 20 m. If your source uses another unit, convert both levels to metres before entering them. If the low-water value is greater than the high-water value, check that the readings are correctly identified and use the appropriate pair of measurements.
How the Tidal Range Calculator formula works
The formula is high-water level − low-water level. Because the two measurements use the same unit and vertical reference, subtracting one from the other gives their vertical separation. The result is displayed in metres with two decimal places.
For a worked example, take a high-water reading of 4.60 m and a low-water reading of 1.35 m. Substitution gives 4.60 m − 1.35 m = 3.25 m. The tidal range between those levels is 3.25 m.
The calculator does not apply a correction, multiplier or location factor. That is intentional: the arithmetic is meaningful only when the entered values already describe comparable levels. If one value is measured relative to a different datum, the subtraction may produce a number that looks precise but does not represent the true separation.
Use cases for the Tidal Range Calculator
A simple range calculation can help organize information for several kinds of marine work. It can be used to:
- Compare the vertical span between high and low water readings in a tide table.
- Summarize observed water levels from a gauge or field log.
- Check a handwritten calculation when reviewing a passage plan or survey notes.
- Communicate the difference between two recorded levels in a consistent unit.
- Prepare a basic comparison of level pairs from the same location and datum.
In each case, the usefulness of the result depends on the source and context of the readings. A range calculated from predicted levels summarizes those predictions. A range calculated from observations summarizes those observations. The result does not independently verify the source values or establish that conditions will repeat.
Other factors to consider when calculating tidal range
Use measurements that share a chart datum or other clearly defined vertical reference. Nautical charts and tide publications may express heights relative to a local datum. A level referenced to a different origin cannot be compared directly unless the relationship between the datums is known and properly accounted for.
Keep the location and time context with the result. Tidal levels vary by place and time, and two readings should represent a meaningful high-to-low pair for the question you are asking. A value from one harbour should not be treated as the range at another. Likewise, a range from one date does not guarantee the same range on a later date.
Range is also different from tidal current. The calculator gives a vertical distance, while current describes horizontal water movement. It cannot determine current direction or speed, navigable depth, under-keel clearance, or a safe departure time. Those questions require appropriate charts, tide and current information, vessel details and local conditions.
Check the precision of the source before relying on displayed decimal places. The calculator formats the answer to two decimals, but that formatting does not make the original measurements more accurate. Record the source, datum, units and observation or prediction time when the result will be used in a log, report or planning discussion.
Frequently asked questions
What does tidal range mean?
Tidal range is the vertical difference between a high-water level and a low-water level for a stated place and context. It is expressed as a distance, such as metres.
Can I enter feet instead of metres?
This calculator is configured for metres. Convert both readings to metres before entering them, and use the same conversion for each value. Do not mix feet and metres in one calculation.
Does this calculator predict the next tide?
No. It subtracts one supplied level from another. A tide prediction requires suitable location-specific data and a time reference, which this calculator does not process.
Why must both levels use the same datum?
A datum is the reference from which a water level is measured. If the readings use different references, their numerical difference may not equal the physical vertical separation. Use comparable levels or obtain a reliable datum relationship.
Is tidal range the same as tidal current?
No. Tidal range is the vertical change between water levels. Tidal current is the horizontal flow of water. A range value alone cannot establish current speed, direction or navigational safety.