diff --git a/packages/mermaid/src/diagrams/sequence/svgDraw.js b/packages/mermaid/src/diagrams/sequence/svgDraw.js
index ce1caea69..220db2209 100644
--- a/packages/mermaid/src/diagrams/sequence/svgDraw.js
+++ b/packages/mermaid/src/diagrams/sequence/svgDraw.js
@@ -1,5 +1,6 @@
 import common from '../common/common';
 import { addFunction } from '../../interactionDb';
+import { parseFontSize } from '../../utils';
 import { sanitizeUrl } from '@braintree/sanitize-url';
 
 export const drawRect = function (elem, rectData) {
@@ -156,6 +157,8 @@ export const drawText = function (elem, textData) {
     textHeight = 0;
   const lines = textData.text.split(common.lineBreakRegex);
 
+  const [_textFontSize, _textFontSizePx] = parseFontSize(textData.fontSize);
+
   let textElems = [];
   let dy = 0;
   let yfunc = () => textData.y;
@@ -218,9 +221,9 @@ export const drawText = function (elem, textData) {
     if (
       textData.textMargin !== undefined &&
       textData.textMargin === 0 &&
-      textData.fontSize !== undefined
+      _textFontSize !== undefined
     ) {
-      dy = i * textData.fontSize;
+      dy = i * _textFontSize;
     }
 
     const textElem = elem.append('text');
@@ -235,8 +238,8 @@ export const drawText = function (elem, textData) {
     if (textData.fontFamily !== undefined) {
       textElem.style('font-family', textData.fontFamily);
     }
-    if (textData.fontSize !== undefined) {
-      textElem.style('font-size', textData.fontSize);
+    if (_textFontSizePx !== undefined) {
+      textElem.style('font-size', _textFontSizePx);
     }
     if (textData.fontWeight !== undefined) {
       textElem.style('font-weight', textData.fontWeight);
@@ -840,8 +843,7 @@ const _drawTextCandidateFunc = (function () {
   function byTspan(content, g, x, y, width, height, textAttrs, conf) {
     const { actorFontSize, actorFontFamily, actorFontWeight } = conf;
 
-    let _actorFontSize =
-      actorFontSize && actorFontSize.replace ? actorFontSize.replace('px', '') : actorFontSize;
+    const [_actorFontSize, _actorFontSizePx] = parseFontSize(actorFontSize);
 
     const lines = content.split(common.lineBreakRegex);
     for (let i = 0; i < lines.length; i++) {
@@ -851,7 +853,7 @@ const _drawTextCandidateFunc = (function () {
         .attr('x', x + width / 2)
         .attr('y', y)
         .style('text-anchor', 'middle')
-        .style('font-size', actorFontSize)
+        .style('font-size', _actorFontSizePx)
         .style('font-weight', actorFontWeight)
         .style('font-family', actorFontFamily);
       text
diff --git a/packages/mermaid/src/diagrams/sequence/svgDraw.spec.js b/packages/mermaid/src/diagrams/sequence/svgDraw.spec.js
index 8e5f5f32b..ed60285ed 100644
--- a/packages/mermaid/src/diagrams/sequence/svgDraw.spec.js
+++ b/packages/mermaid/src/diagrams/sequence/svgDraw.spec.js
@@ -125,6 +125,30 @@ describe('svgDraw', function () {
       expect(text3.attr).toHaveBeenCalledWith('y', 10);
       expect(text3.text).toHaveBeenCalledWith('fine lines');
     });
+    it('should work with numeral font sizes', function () {
+      const svg = MockD3('svg');
+      svgDraw.drawText(svg, {
+        x: 10,
+        y: 10,
+        dy: '1em',
+        text: 'One fine text message',
+        class: 'noteText',
+        fontFamily: 'courier',
+        fontSize: 10,
+        fontWeight: '500',
+      });
+      expect(svg.__children.length).toBe(1);
+      const text = svg.__children[0];
+      expect(text.__name).toBe('text');
+      expect(text.attr).toHaveBeenCalledWith('x', 10);
+      expect(text.attr).toHaveBeenCalledWith('y', 10);
+      expect(text.attr).toHaveBeenCalledWith('dy', '1em');
+      expect(text.attr).toHaveBeenCalledWith('class', 'noteText');
+      expect(text.text).toHaveBeenCalledWith('One fine text message');
+      expect(text.style).toHaveBeenCalledWith('font-family', 'courier');
+      expect(text.style).toHaveBeenCalledWith('font-size', '10px');
+      expect(text.style).toHaveBeenCalledWith('font-weight', '500');
+    });
   });
   describe('drawBackgroundRect', function () {
     it('should append a rect before the previous element within a given bound', function () {
diff --git a/packages/mermaid/src/utils.spec.js b/packages/mermaid/src/utils.spec.js
index 7ee6aa000..0f0bc1e92 100644
--- a/packages/mermaid/src/utils.spec.js
+++ b/packages/mermaid/src/utils.spec.js
@@ -402,3 +402,29 @@ describe('when inserting titles', function () {
     expect(titleAttrSpy).toHaveBeenCalledWith('class', 'testClass');
   });
 });
+
+describe('when parsing font sizes', function () {
+  it('parses number inputs', function () {
+    expect(utils.parseFontSize(14)).toEqual([14, '14px']);
+  });
+
+  it('parses string em inputs', function () {
+    expect(utils.parseFontSize('14em')).toEqual([14, '14em']);
+  });
+
+  it('parses string px inputs', function () {
+    expect(utils.parseFontSize('14px')).toEqual([14, '14px']);
+  });
+
+  it('parses string inputs without units', function () {
+    expect(utils.parseFontSize('14')).toEqual([14, '14px']);
+  });
+
+  it('handles undefined input', function () {
+    expect(utils.parseFontSize(undefined)).toEqual([undefined, undefined]);
+  });
+
+  it('handles unparseable input', function () {
+    expect(utils.parseFontSize({ fontSize: 14 })).toEqual([undefined, undefined]);
+  });
+});
diff --git a/packages/mermaid/src/utils.ts b/packages/mermaid/src/utils.ts
index 4c2f844e2..876c81543 100644
--- a/packages/mermaid/src/utils.ts
+++ b/packages/mermaid/src/utils.ts
@@ -543,12 +543,14 @@ export const drawSimpleText = function (
   // Remove and ignore br:s
   const nText = textData.text.replace(common.lineBreakRegex, ' ');
 
+  const [, _fontSizePx] = parseFontSize(textData.fontSize);
+
   const textElem = elem.append('text');
   textElem.attr('x', textData.x);
   textElem.attr('y', textData.y);
   textElem.style('text-anchor', textData.anchor);
   textElem.style('font-family', textData.fontFamily);
-  textElem.style('font-size', textData.fontSize);
+  textElem.style('font-size', _fontSizePx);
   textElem.style('font-weight', textData.fontWeight);
   textElem.attr('fill', textData.fill);
   if (textData.class !== undefined) {
@@ -722,6 +724,8 @@ export const calculateTextDimensions: (
       return { width: 0, height: 0 };
     }
 
+    const [, _fontSizePx] = parseFontSize(fontSize);
+
     // We can't really know if the user supplied font family will render on the user agent;
     // thus, we'll take the max width between the user supplied font family, and a default
     // of sans-serif.
@@ -745,7 +749,7 @@ export const calculateTextDimensions: (
         const textObj = getTextObj();
         textObj.text = line;
         const textElem = drawSimpleText(g, textObj)
-          .style('font-size', fontSize)
+          .style('font-size', _fontSizePx)
           .style('font-weight', fontWeight)
           .style('font-family', fontFamily);
 
@@ -941,6 +945,32 @@ export const insertTitle = (
     .attr('class', cssClass);
 };
 
+/**
+ * Parses a raw fontSize configuration value into a number and string value.
+ *
+ * @param fontSize - a string or number font size configuration value
+ *
+ * @returns parsed number and string style font size values, or nulls if a number value can't
+ * be parsed from an input string.
+ */
+export const parseFontSize = (fontSize: string | number | undefined): [number?, string?] => {
+  // if the font size is a number, assume a px string representation
+  if (typeof fontSize === 'number') {
+    return [fontSize, fontSize + 'px'];
+  }
+
+  const fontSizeNumber = parseInt(fontSize, 10);
+  if (Number.isNaN(fontSizeNumber)) {
+    // if a number value can't be parsed, return null for both values
+    return [undefined, undefined];
+  } else if (fontSize === String(fontSizeNumber)) {
+    // if a string input doesn't contain any units, assume px units
+    return [fontSizeNumber, fontSize + 'px'];
+  } else {
+    return [fontSizeNumber, fontSize];
+  }
+};
+
 export default {
   assignWithDepth,
   wrapLabel,
@@ -964,4 +994,5 @@ export default {
   directiveSanitizer,
   sanitizeCss,
   insertTitle,
+  parseFontSize,
 };