Enter coordinates from two points on the linear portion of your graph.
Point 1 (Start)
Minutes (X-axis)
mL or mm (Y-axis)
Point 2 (End)
Minutes (X-axis)
mL or mm (Y-axis)
Optional Parameters
Leave blank if calculating total uptake only.
Change in Volume/Distance (Δy):–
Time Elapsed (Δx):–
Rate of Transpiration (Slope):–
Rate per Unit Area:–
function calculateTranspiration() {
// Clear errors and results
document.getElementById('errorMsg').style.display = 'none';
document.getElementById('resultBox').style.display = 'none';
document.getElementById('areaRow').style.display = 'none';
// Get Input Values
var t1 = parseFloat(document.getElementById('time1').value);
var v1 = parseFloat(document.getElementById('reading1').value);
var t2 = parseFloat(document.getElementById('time2').value);
var v2 = parseFloat(document.getElementById('reading2').value);
var area = parseFloat(document.getElementById('leafArea').value);
// Validation
if (isNaN(t1) || isNaN(v1) || isNaN(t2) || isNaN(v2)) {
var errorDiv = document.getElementById('errorMsg');
errorDiv.innerHTML = "Please enter valid numbers for both coordinate points.";
errorDiv.style.display = 'block';
return;
}
if (t1 === t2) {
var errorDiv = document.getElementById('errorMsg');
errorDiv.innerHTML = "Time 1 and Time 2 cannot be the same (division by zero).";
errorDiv.style.display = 'block';
return;
}
// Calculation Logic: Slope formula m = (y2 – y1) / (x2 – x1)
var deltaX = t2 – t1;
var deltaY = v2 – v1;
// Transpiration is usually positive, but potometer readings might go down.
// We calculate absolute rate usually, or simple slope.
// Let's assume the user enters points such that positive slope = uptake.
// If the graph shows volume remaining (decreasing), slope is negative.
// We will display the raw slope.
var rate = deltaY / deltaX;
// Update DOM
document.getElementById('deltaYResult').innerHTML = deltaY.toFixed(3) + " units";
document.getElementById('deltaXResult').innerHTML = deltaX.toFixed(2) + " min";
document.getElementById('slopeResult').innerHTML = rate.toFixed(4) + " units/min";
// Handle Area Calculation
if (!isNaN(area) && area > 0) {
var ratePerArea = rate / area;
document.getElementById('areaResult').innerHTML = ratePerArea.toFixed(6) + " units/min/cm²";
document.getElementById('areaRow').style.display = 'flex';
}
document.getElementById('resultBox').style.display = 'block';
}
How to Calculate Rate of Transpiration from a Graph
In biology experiments, particularly when using a potometer, the rate of transpiration is determined by measuring water uptake over time. The data is typically plotted on a graph with Time on the X-axis and Distance Moved (meniscus) or Volume of Water on the Y-axis.
The rate of transpiration is mathematically equivalent to the gradient (slope) of the line of best fit on this graph.
1. Identify the Linear Phase
Transpiration experiments may take a few minutes to stabilize. When calculating the rate, ignore the initial inconsistent data points. Look for the section of the graph where the line is straight (linear). This indicates a constant rate of transpiration.
2. The Gradient Formula
To calculate the rate from the graph, you select two points on the straight line: Point A $(x_1, y_1)$ and Point B $(x_2, y_2)$. The formula is:
Rate = Change in Y / Change in X
Rate = (y₂ – y₁) / (x₂ – x₁)
Change in Y: The volume of water absorbed (mL) or distance the bubble moved (mm).
Change in X: The duration of time elapsed (minutes).
3. Units of Measurement
The resulting unit depends on your measuring instrument:
If measuring bubble movement distance: mm/min.
If measuring volume (graduated pipette): mL/min or µL/min.
4. Accounting for Leaf Surface Area
To compare transpiration rates between different plant species or different leaves, you must normalize the data by dividing the rate by the total surface area of the leaves. This results in a rate expressed as mL/min/cm².
This calculator allows you to input coordinates directly from your lab graph to instantly find the slope and the normalized rate per unit area.