Friction factor is used when calculating head loss due to friction (i.e. Darcy's equation) in pressurized pipes or ducts.
<h1>Moody's Friction Factor Calculator</h1>
<form name="friction_factor" style="background-color:#E6E6FA"><script type="text/javascript"> 
	function calculate_f(){
		var Re   = Number(document.friction_factor.ReynoldsNum.value); //Reynolds number
		var RR = Number(document.friction_factor.RR.value); //Relative roughness
		document.friction_factor.ff.value=friction_factor_moody(Re,RR); //Call JS function to calculate friction factor using Colebrook Equation
function friction_factor_moody(R, K) {
	  if (R <= 2300) {
		return 64 / R;
	  } else if (R <= 4000) {
		return "Transition";
	  } else {
		f_guess = 0.02;
		f_final = 0.01;
		iter = 0;
		while (iter < 100) {
		  if (f_final.toFixed(4) != f_guess.toFixed(4)) {
			f_guess = f_final;
			A = -2 * Math.log10((K / 3.7) + (2.51 / (R * Math.sqrt(f_guess))));
			f_final = Math.pow((1 / A), 2);
		  } else {
			break;
		  }
		  iter++;
		}
		if (iter == 100) {
		  return "Not converged"
		} else {
		  return f_final;
		}
	  }
	}
	}
</script>
<table align="center">
<tbody>
<tr>
<td align="right">Reynold's number $(Re)$:</td>
<td><input name="ReynoldsNum" type="text" /></td>
</tr>
<tr>
<td align="right">Relative Roughness $(\frac{\epsilon}{D}$ or $\frac{k_s}{D})$:</td>
<td><input name="RR" type="text" /></td>
</tr>
<tr align="center">
<td colspan="2"><input type="button" value="Calculate" onclick="javascript:calculate_f();"/></td>
</tr>
<tr>
<td align="right">Friction Factor $(f)$:</td>
<td><input name="ff" readonly="readonly" type="text" /></td>
</tr>
</tbody>
</table>
</form>This calculator uses an iterative procedure to solve the <em>Colebrook equation</em> for computing friction factor $(f)$ when flows are <em>fully turbulent</em>, i.e. $Re \geq 4000$:
[latexpage]
<p style="text-align: center;">$\frac{1}{\sqrt[]{f}} = -2.0 log( \frac{\epsilon}{3.7D} + \frac{2.51}{Re\sqrt[]{f}})$</p>
where, $f$ is the <em>friction factor</em>, $\frac{\epsilon}{D}$ is the <em>relative roughness</em>, and $Re$ is the <em>Reynold's number</em>.
When the flow is <em>Laminar</em>, i.e. $Re \leq 2300$, the friction factor is given by:
<p style="text-align: center;">$f=\frac{64}{Re}$</p>
For <em>transition flows</em>, i.e. $ 2300 < Re \leq 4000$, the Colebrook equation is not applicable. Therefore, a numeric solution is not provided by this tool. In such a situation, users may choose to interpolate the value of friction factor between the <em>laminar</em> $(Re = 2300)$ and <em>turbulent</em> $(Re = 4000)$ values.

The figure below shows the Moody diagram.
<img src="https://www.gmallya.com/wp-content/uploads/2018/04/Moodys-chart-v3.jpg" alt="Moody Diagram Friction Factor" width="800" height="498" class="alignnone size-full wp-image-141" />

If you use this calculator for your school or academic work, I encourage you to cite this tool as follows:
<em>Mallya, Ganeshchandra. “Moody's Friction Factor Calculator.” Towards Open Science, 03/31/2018, www.gmallya.com/moodys-friction-factor-calculator.</em>

The tool is released under MIT license, i.e. the tool is provided "AS IS", without warranty of any kind. The code is made available in the <a href="https://github.com/gcmallya85/Moodys_Calculator" rel="noopener" target="_blank">GitHub repository</a>. If you have suggestions for improvement to the tool or the write-up leave a comment and I will get back to you at the earliest.