Mathematics · Algebra
Proportion Calculator
Solve a proportional relationship a/b = c/x by cross multiplication.
Inputs and results stay in this browser. Change one value at a time to explore the relationship.
Calculation steps
- Cross multiply: 2x = 5 × 8.
- 2x = 40.
- x = 40 ÷ 2 = 20.
Understand Proportion solver
One idea, three depths
Choose how deeply to explain Proportion solver
Proportion solver: Solve a proportional relationship a/b = c/x by cross multiplication.
Age 5Explain it to a 5-year-oldStart with a picture
Imagine using Proportion solver to answer this question: solve a proportional relationship a/b = c/x by cross multiplication? Enter a, b, c; the calculator shows Missing value x. For example: For 2/5 = 8/x, x = 5 × 8 ÷ 2 = 20. The answer tells you Missing value x.
Age 15Explain it to a 15-year-oldConnect it to the formula
Equal ratios have equal cross-products, so ax = bc and division by a isolates x. The rule is x = bc ÷ a. Its input values are a, b, c, and the main result is Missing value x. For example: For 2/5 = 8/x, x = 5 × 8 ÷ 2 = 20.
CollegeExplain it at college levelState the model precisely
This calculator evaluates the stated proportion solver relation over the valid real-number domain stated below. The implemented relation is x = bc ÷ a, evaluated from a, b, c to produce Missing value x. Equal ratios have equal cross-products, so ax = bc and division by a isolates x. Keep corresponding quantities in the same position in both ratios.
Inputs and valid domain
- a must be a finite real number.
- b must be a finite real number.
- c must be a finite real number.
Important boundary: Keep corresponding quantities in the same position in both ratios.
The formula
x = bc ÷ a
How the calculator works through it
It substitutes a, b, c into the formula and exposes every numerical step above. The main output is Missing value x.
Read the result correctly
The Missing value x is the direct answer to “solve a proportional relationship a/b = c/x by cross multiplication.” Read it with the units shown beside the inputs; a sign, angle, percentage or rate changes what the number means.
A worked check
For 2/5 = 8/x, x = 5 × 8 ÷ 2 = 20.
Where this model stops being reliable
Keep corresponding quantities in the same position in both ratios.
Learn it by changing one value
Begin with the worked example, then change one value while keeping the others fixed. Compare the new result and calculation steps to identify which part of the formula changed.
Dictionary terms behind this calculator
Before studying the codeWhat you should know firstUse the calculator immediately, or check the foundations before reading the implementation.
These foundations help you understand why Proportion solver works. They never block the calculator, and “optional” means useful context rather than a hidden requirement.
Hard requirements
- Reading formulas and substituting values
Proportion solver uses x = bc ÷ a. You need to recognise what each side represents before substituting the stated inputs or rearranging the relationship.
Review this foundation about 4 min
Strong support
- Functions and input-output rules
A function viewpoint helps you see how changing an input changes the Proportion solver result.
Review this foundation about 5 min
Optional enrichment
- Powers and exponents
Powers are not required for every Proportion solver calculation, but they make related algebraic forms and code easier to read.
Review this foundation about 4 min
Mathematics → algorithm → program
Implement this calculation in code
These are direct reference implementations of the calculator's principal relationship and first output. They run locally and include a small known-answer check where the language supports it.
Algorithm
- Read a, b, c.
- Evaluate the principal relationship: x = bc ÷ a.
- Return Missing value x and check the domain conditions described above.
Python
from math import *
def proportion(a, b, c) -> float:
return ((b * c) / a)
assert abs(proportion(2, 5, 8) - 20) < 1e-6 * max(1.0, abs(20))
C
#include <assert.h>
#include <math.h>
double proportion(double a, double b, double c) {
return ((b * c) / a);
}
int main(void) {
const double expected = 20;
const double actual = proportion(2, 5, 8);
assert(fabs(actual - expected) < 1e-6 * fmax(1.0, fabs(expected)));
}
C++
#include <cassert>
#include <cmath>
#include <numbers>
double proportion(double a, double b, double c) {
return ((b * c) / a);
}
int main() {
constexpr double expected = 20;
const double actual = proportion(2, 5, 8);
assert(std::fabs(actual - expected) < 1e-6 * std::fmax(1.0, std::fabs(expected)));
}
Linux x86-64 assembly
x86-64 NASM · System V ABI · Linux · SSE2 with libm where required
; double proportion(double a, double b, double c)
; Linux x86-64 NASM · System V ABI · first eight doubles in xmm0–xmm7
global proportion
section .text
proportion:
push rbp
mov rbp, rsp
sub rsp, 48
movsd [rbp-8], xmm0
movsd [rbp-16], xmm1
movsd [rbp-24], xmm2
movsd xmm0, [rbp-16]
mulsd xmm0, [rbp-24]
movsd [rbp-40], xmm0
movsd xmm0, [rbp-40]
divsd xmm0, [rbp-8]
movsd [rbp-32], xmm0
movsd xmm0, [rbp-32]
leave
ret
MATLAB
function result = proportion(a, b, c)
result = ((b * c) / a);
end
Wolfram Language
ClearAll[mwCalculate];
mwCalculate[a_, b_, c_] := ((b * c) / a);
Continue in mathematical software
The downloaded file includes your current inputs and first calculated result. It is created locally.
Floating-point answers can differ slightly by language, compiler and processor. Compare within a suitable tolerance rather than assuming every decimal representation will be identical.
Supporting sourcesAcademic referencesPrimary standards, textbooks and complete citations
Standards, reading and academic references
Use the calculator as the worked interaction, then consult the primary standards and academic textbooks listed below. MW SysArc links to the original sources; the explanation on this page is original and does not reproduce them.
Algebra and Trigonometry 2e
Read the related free OpenStax mathematics chaptersCite this book
- APA 7
- Abramson, J. (2021). Algebra and trigonometry 2e. OpenStax. https://openstax.org/books/algebra-and-trigonometry-2e/pages/1-introduction-to-prerequisites
- MLA 9
- Abramson, Jay. Algebra and Trigonometry 2e. OpenStax, 2021, https://openstax.org/books/algebra-and-trigonometry-2e/pages/1-introduction-to-prerequisites.
- Chicago author-date
- Abramson, Jay. 2021. Algebra and Trigonometry 2e. Houston, TX: OpenStax. https://openstax.org/books/algebra-and-trigonometry-2e/pages/1-introduction-to-prerequisites.
OpenStax entries are free to read online. Follow the licence shown on each linked source before redistributing or adapting its content.
Reuse the page responsiblyCite this pageAPA, MLA, Chicago, Harvard, BibTeX and RIS
These formats cite this calculator page itself. They are separate from the academic references above, which support the mathematical method and terminology.
APA 7
MW SysArc. (2026, July 21). Proportion Calculator. MW SysArc Tools. https://math.mwsysarc.com/algebra/proportion-calculator
MLA 9
MW SysArc. “Proportion Calculator.” MW SysArc Tools, 21 July 2026, https://math.mwsysarc.com/algebra/proportion-calculator. Accessed 31 Aug. 2026.
Chicago 17
MW SysArc. “Proportion Calculator.” MW SysArc Tools. Published July 21, 2026. Accessed August 31, 2026. https://math.mwsysarc.com/algebra/proportion-calculator.
Harvard
MW SysArc (2026) ‘Proportion Calculator’, MW SysArc Tools. Published 21 July 2026. Available at: https://math.mwsysarc.com/algebra/proportion-calculator (Accessed: 31 August 2026).
BibTeX and RIS records
BibTeX
@misc{mwsysarc_proportion_2026,
author = {{MW SysArc}},
title = {Proportion Calculator},
howpublished = {MW SysArc Tools},
year = {2026},
url = {https://math.mwsysarc.com/algebra/proportion-calculator},
note = {Published July 21, 2026; accessed August 31, 2026}
}RIS
TY - ELEC
AU - MW SysArc
TI - Proportion Calculator
T2 - MW SysArc Tools
PY - 2026
DA - 2026-07-21
Y2 - 2026-08-31
UR - https://math.mwsysarc.com/algebra/proportion-calculator
N1 - Published July 21, 2026
ER -Clear answers
Frequently asked questions
What does the Proportion solver do?
Solve a proportional relationship a/b = c/x by cross multiplication.
How does the Proportion solver work?
The calculator applies x = bc ÷ a. Equal ratios have equal cross-products, so ax = bc and division by a isolates x.
What can I learn from the Proportion solver?
It connects the mathematical rule to your chosen numbers and shows each calculation step. Change one input at a time to see how the result responds.
Does MW SysArc receive or store what I enter?
No. The calculation runs locally in your browser. MW SysArc does not receive or store your calculation inputs.
How should I use the result?
Use the steps to understand the method, then verify important school or professional work using the notation and rounding rules required in your setting.
Last reviewed . Calculations tested .