Home› Calculators› Ratio Calculator
Free · 5 Modes · Steps Shown

RatioCalculator

Simplify ratios, solve proportions, find missing values, calculate debt-to-income ratio, aspect ratio, and scale ratio — all with step-by-step solutions. Free, instant, no signup.

✓ 100% Free Simplify Ratio Proportion Solver DTI Ratio Aspect Ratio
Ratio Calculator
✦ Steps Included
Enter any two numbers to reduce the ratio A:B to its simplest form using GCD.
A
:
B
Please enter valid positive numbers.
5-in-1
Ratio Modes
Steps
Always Shown
DTI
Mortgage-Ready
Free
Forever
Step-by-Step Guide

How to Use the Ratio Calculator

Five modes cover every ratio calculation. Select the right mode, enter your numbers, and get a complete step-by-step solution instantly.

01
Choose a Mode

Select Simplify to reduce A:B, Proportion to find a missing value, DTI for debt-to-income, Aspect Ratio for screen sizes, or Scale for map distances.

02
Enter Your Numbers

Type the values into the clearly labelled fields. For DTI, enter each monthly debt payment separately and your gross income. For Proportion, leave the unknown blank.

03
Click Calculate

Press Calculate Ratio and see your result instantly — shown in bold with the simplified ratio, decimal equivalent, and all working steps clearly listed.

04
Read the Steps

Every result shows GCD calculation, simplification steps, cross-multiplication for proportions, or DTI zone assessment — so you understand the full method.

Debt-to-Income Guide

What Is a Good DTI Ratio?

Your debt-to-income ratio tells lenders how much of your income is already committed to debt. Here's what the numbers mean for mortgage approval and financial health.

Debt to income ratio calculator — mortgage approval DTI calculation on financial documents
Debt-to-income ratio calculator — understand your DTI for mortgage approval and financial planning
Under 20%
Excellent
Very healthy. Easily qualify for loans at best rates. Lots of financial flexibility.
20% – 36%
Good
Lenders consider this good. Most mortgages approved. Room for more credit if needed.
37% – 49%
Manageable
Some lenders approve. FHA max is 43%. Consider reducing debt before applying for mortgage.
50%+
High Risk
Most lenders decline at this level. Focus on paying down debt before applying for loans.
Types of Ratios

Common Ratio Types

Ratios appear in finance, health, engineering, photography, and everyday life. Here are the most commonly calculated ratio types our calculator covers.

Debt-to-Income
DTI = Debt / Income × 100

Used for mortgage qualification, loan approval, and financial health assessment.

Aspect Ratio
AR = Width : Height

Used in screens, photography, video (16:9, 4:3, 1:1) and responsive web design.

Gear Ratio
GR = Driver Teeth / Driven Teeth

Used in mechanical engineering, bicycles, and automotive transmissions.

Cholesterol Ratio
Total / HDL Cholesterol

Heart disease risk indicator. Below 5:1 is healthy; below 3.5:1 is ideal.

Waist-to-Hip Ratio
WHR = Waist / Hip

Health risk indicator. For women: <0.85 healthy. For men: <0.90 healthy.

Scale Ratio
1 : n (map to real size)

Used in architecture, map reading, model building, and engineering drawings.

Who Uses This

Real-Life Use Cases

Ratios are fundamental to finance, health, design, cooking, engineering, and science. Here's where our free ratio calculator helps most.

Mortgage & Loans

Calculate DTI ratio before applying for a mortgage. Most lenders require DTI below 36-43%. Know your ratio before the bank does.

Photography & Video

Find the correct aspect ratio for YouTube thumbnails (16:9), Instagram posts (1:1), or cinema (2.39:1). Scale dimensions precisely.

Map Reading & Scale

Convert map distances to real distances using scale ratios. 1:50,000 means 1cm on map = 50,000cm = 500m in reality.

Cooking & Recipes

Scale recipes up or down using proportions. If a recipe serves 4 and you need 10, use our proportion solver to find the new ingredient quantities.

Health & Medicine

Calculate waist-to-hip ratio, cholesterol ratio, BMI comparisons, medication dilution ratios, and PF ratio (lung oxygen efficiency).

Business & Finance

Calculate expense ratio, rent-to-income ratio, profit-to-cost ratio, power-to-weight ratio for products, and dilution ratios for chemical mixing.

What Is a Ratio Calculator?

A ratio calculator is a free online tool that simplifies ratios to their lowest terms, solves proportions to find missing values, and calculates specialized ratios like the debt-to-income ratio, aspect ratio, and scale ratio — all with step-by-step working shown. Our ratio calculator online covers five different ratio modes in one place, making it far more versatile than basic ratio tools that only simplify.

How to Simplify a Ratio — Step by Step

To simplify a ratio A:B, divide both numbers by their Greatest Common Divisor (GCD). The GCD is the largest number that divides both A and B without a remainder.

📘 Example — Simplify 24:36

Factors of 24: 1, 2, 3, 4, 6, 8, 12, 24
Factors of 36: 1, 2, 3, 4, 6, 9, 12, 18, 36
GCD(24, 36) = 12
24 ÷ 12 = 2  |  36 ÷ 12 = 3
Simplified ratio: 2:3

Ratio calculator online — proportion solving, aspect ratio, and scale ratio calculations
Online ratio calculator — simplify ratios, solve proportions, aspect ratio, scale ratio, and DTI calculations

What Is My Debt to Income Ratio?

Your debt-to-income (DTI) ratio is the percentage of your gross monthly income (before taxes) that goes toward paying your monthly debts. It is one of the most important numbers lenders look at when you apply for a mortgage, car loan, or any other credit.

Formula: DTI = (Total Monthly Debt Payments ÷ Gross Monthly Income) × 100

How to Calculate Debt to Income Ratio

  1. Add up ALL monthly debt payments: mortgage/rent, car loans, student loans, credit card minimum payments, personal loans, child support, alimony
  2. Find your gross monthly income (your salary before tax, plus any other regular income)
  3. Divide total monthly debt by gross monthly income
  4. Multiply by 100 to get the percentage

📘 Example — How to Figure Out Debt to Income Ratio

Mortgage: $1,200/month
Car loan: $350/month
Student loan: $200/month
Credit cards: $150/month
Total debt: $1,900/month

Gross monthly income: $5,500
DTI = ($1,900 ÷ $5,500) × 100 = 34.5%
Assessment: Good — most lenders will approve

What is a good debt-to-income ratio? For mortgage approval, aim for a DTI below 36%. Below 20% is considered excellent. For FHA loans, the limit is typically 43%. For conventional loans, most lenders prefer 36% or less. Our DTI calculator shows your zone (Excellent / Good / Manageable / High Risk) automatically.

How to Solve a Proportion (Missing Value)

A proportion states that two ratios are equal: A:B = C:D. If any one value is missing, you can find it using cross-multiplication:

A × D = B × C

📘 Example — Find D: 3:4 = 9:?

Cross multiply: 3 × D = 4 × 9
3D = 36
D = 36 ÷ 3 = 12
Answer: 3:4 = 9:12 ✓ (both simplify to 3:4)

Aspect Ratio Calculator — Screen & Photo Sizes

The aspect ratio describes the proportional relationship between width and height. It is written as W:H. Common aspect ratios:

Aspect ratio calculator and scale ratio — used in photography, video production, and architecture
Ratio calculator online — aspect ratio for screens, scale ratio for maps and blueprints, DTI for finance

Rent to Income Ratio — What Is Affordable?

The common rule is that rent should not exceed 30% of your gross monthly income. This is calculated as: Rent Ratio = Monthly Rent ÷ Monthly Income × 100

Example: Rent of $1,500 / Income of $5,000 = 30% — right at the affordability limit. Use our Proportion mode to calculate this by entering: rent as A, income as B, 30 as C, and find D (the maximum rent you can afford at 30%).

Common Questions

Ratio Calculator FAQ

What is a debt-to-income ratio and how to calculate it?

The debt-to-income (DTI) ratio is the percentage of your gross monthly income that goes to debt payments. Formula: DTI = (Total Monthly Debt ÷ Gross Monthly Income) × 100. Include all monthly payments: mortgage, car, student loans, credit cards, personal loans. Use our DTI tab — enter each debt and your income, then click Calculate to get your DTI% and lender assessment instantly.

What is a good debt-to-income ratio for a mortgage?

For mortgage approval: Under 36% is ideal and qualifies for most conventional loans. 36-43% is acceptable — FHA loans allow up to 43%. 43-49% is risky and limits your options. 50% or above — most lenders will decline. The lower your DTI, the better your interest rate. Aim to reduce debts before applying if your DTI is above 36%.

How to simplify a ratio — what is the ratio calculator method?

Enter A and B into the Simplify tab and click Calculate. The calculator finds the Greatest Common Divisor (GCD) of both numbers, then divides both by it: Simplified = (A÷GCD) : (B÷GCD). For example, 18:12 → GCD=6 → 3:2. The result also shows the decimal form (18:12 = 1.5:1) and percentage split (60% : 40%).

How to find a missing value in a proportion?

Use our Proportion tab. Select which value (A, B, C, or D) is missing, enter the three known values, and click Calculate. The method is cross-multiplication: in A:B = C:D, if D is missing: D = (B × C) ÷ A. Example: 5:8 = 15:? → ? = (8 × 15) ÷ 5 = 24. Works for all four positions.

How to calculate aspect ratio for a new screen size?

Use the Aspect Ratio tab. Enter the original Width and Height (e.g. 1920 × 1080), then optionally enter a new Width to calculate the proportional Height. The calculator simplifies the ratio using GCD (1920:1080 → 16:9) and scales dimensions proportionally. This ensures images and videos are not distorted when resizing.

What is the rent-to-income ratio rule?

The standard rule is that rent should be no more than 30% of your gross monthly income. This is the rent-to-income ratio. Formula: Rent Ratio = (Monthly Rent ÷ Monthly Income) × 100. If you earn $4,000/month, maximum affordable rent is $1,200 (30%). Use our DTI calculator — enter rent as a debt payment — to see how rent affects your overall DTI ratio.

More Free Tools

Related Calculators

Percentage Calculator
6 percentage modes
Fraction Calculator
Simplify & convert
Average Calculator
Mean, median, mode
Scientific Calculator
Full math functions