diff --git a/web/utils/tooltip-utils.js b/web/utils/tooltip-utils.js
index 120133029..05cf518b8 100644
--- a/web/utils/tooltip-utils.js
+++ b/web/utils/tooltip-utils.js
@@ -1,356 +1,403 @@
 // @flow
 
 import invariant from 'invariant';
 import * as React from 'react';
 
 import { getAppContainerPositionInfo } from './window-utils.js';
 import {
   tooltipButtonStyle,
   tooltipLabelStyle,
   tooltipStyle,
+  reactionTooltipStyle,
+  reactionSeeMoreLabel,
 } from '../chat/chat-constants.js';
 import type { PositionInfo } from '../chat/position-types.js';
 import { calculateMaxTextWidth } from '../utils/text-utils.js';
 
 export const tooltipPositions = Object.freeze({
   LEFT: 'left',
   RIGHT: 'right',
   LEFT_BOTTOM: 'left-bottom',
   RIGHT_BOTTOM: 'right-bottom',
   LEFT_TOP: 'left-top',
   RIGHT_TOP: 'right-top',
   TOP: 'top',
   BOTTOM: 'bottom',
 });
 
 export type TooltipSize = {
   +height: number,
   +width: number,
 };
 
 export type TooltipPositionStyle = {
   +anchorPoint: {
     +x: number,
     +y: number,
   },
   +verticalPosition: 'top' | 'bottom',
   +horizontalPosition: 'left' | 'right',
   +alignment: 'left' | 'center' | 'right',
 };
 
 export type TooltipPosition = $Values<typeof tooltipPositions>;
 
 export type MessageTooltipAction = {
   +label: string,
   +onClick: (SyntheticEvent<HTMLDivElement>) => mixed,
   +actionButtonContent: React.Node,
 };
 
 type FindTooltipPositionArgs = {
   +sourcePositionInfo: PositionInfo,
   +tooltipSize: TooltipSize,
   +availablePositions: $ReadOnlyArray<TooltipPosition>,
   +defaultPosition: TooltipPosition,
   +preventDisplayingBelowSource?: boolean,
 };
 
 function findTooltipPosition({
   sourcePositionInfo,
   tooltipSize,
   availablePositions,
   defaultPosition,
   preventDisplayingBelowSource,
 }: FindTooltipPositionArgs): TooltipPosition {
   const appContainerPositionInfo = getAppContainerPositionInfo();
 
   if (!appContainerPositionInfo) {
     return defaultPosition;
   }
 
   const pointingTo = sourcePositionInfo;
   const {
     top: containerTop,
     left: containerLeft,
     right: containerRight,
     bottom: containerBottom,
   } = appContainerPositionInfo;
 
   const tooltipWidth = tooltipSize.width;
   const tooltipHeight = tooltipSize.height;
 
   const canBeDisplayedOnLeft = containerLeft + tooltipWidth <= pointingTo.left;
   const canBeDisplayedOnRight =
     tooltipWidth + pointingTo.right <= containerRight;
 
   const willCoverSidebarOnTopSideways =
     preventDisplayingBelowSource &&
     pointingTo.top + tooltipHeight > pointingTo.bottom;
 
   const canBeDisplayedOnTopSideways =
     pointingTo.top >= containerTop &&
     pointingTo.top + tooltipHeight <= containerBottom &&
     !willCoverSidebarOnTopSideways;
 
   const canBeDisplayedOnBottomSideways =
     pointingTo.bottom <= containerBottom &&
     pointingTo.bottom - tooltipHeight >= containerTop;
 
   const verticalCenterOfPointingTo = pointingTo.top + pointingTo.height / 2;
   const horizontalCenterOfPointingTo = pointingTo.left + pointingTo.width / 2;
 
   const willCoverSidebarInTheMiddleSideways =
     preventDisplayingBelowSource &&
     verticalCenterOfPointingTo + tooltipHeight / 2 > pointingTo.bottom;
 
   const canBeDisplayedInTheMiddleSideways =
     verticalCenterOfPointingTo - tooltipHeight / 2 >= containerTop &&
     verticalCenterOfPointingTo + tooltipHeight / 2 <= containerBottom &&
     !willCoverSidebarInTheMiddleSideways;
 
   const canBeDisplayedOnTop =
     pointingTo.top - tooltipHeight >= containerTop &&
     horizontalCenterOfPointingTo - tooltipWidth / 2 >= containerLeft &&
     horizontalCenterOfPointingTo + tooltipWidth / 2 <= containerRight;
 
   const canBeDisplayedOnBottom =
     pointingTo.bottom + tooltipHeight <= containerBottom &&
     horizontalCenterOfPointingTo - tooltipWidth / 2 >= containerLeft &&
     horizontalCenterOfPointingTo + tooltipWidth / 2 <= containerRight &&
     !preventDisplayingBelowSource;
 
   for (const tooltipPosition of availablePositions) {
     if (
       tooltipPosition === tooltipPositions.RIGHT &&
       canBeDisplayedOnRight &&
       canBeDisplayedInTheMiddleSideways
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.RIGHT_BOTTOM &&
       canBeDisplayedOnRight &&
       canBeDisplayedOnBottomSideways
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.LEFT &&
       canBeDisplayedOnLeft &&
       canBeDisplayedInTheMiddleSideways
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.LEFT_BOTTOM &&
       canBeDisplayedOnLeft &&
       canBeDisplayedOnBottomSideways
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.LEFT_TOP &&
       canBeDisplayedOnLeft &&
       canBeDisplayedOnTopSideways
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.RIGHT_TOP &&
       canBeDisplayedOnRight &&
       canBeDisplayedOnTopSideways
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.TOP &&
       canBeDisplayedOnTop
     ) {
       return tooltipPosition;
     } else if (
       tooltipPosition === tooltipPositions.BOTTOM &&
       canBeDisplayedOnBottom
     ) {
       return tooltipPosition;
     }
   }
   return defaultPosition;
 }
 
 type GetTooltipStyleParams = {
   +sourcePositionInfo: PositionInfo,
   +tooltipSize: TooltipSize,
   +tooltipPosition: TooltipPosition,
 };
 
 // ESLint doesn't recognize that invariant always throws
 // eslint-disable-next-line consistent-return
 function getTooltipStyle({
   sourcePositionInfo,
   tooltipSize,
   tooltipPosition,
 }: GetTooltipStyleParams): TooltipPositionStyle {
   if (tooltipPosition === tooltipPositions.RIGHT_TOP) {
     return {
       anchorPoint: {
         x: sourcePositionInfo.right,
         y: sourcePositionInfo.top,
       },
       horizontalPosition: 'right',
       verticalPosition: 'bottom',
       alignment: 'left',
     };
   } else if (tooltipPosition === tooltipPositions.LEFT_TOP) {
     return {
       anchorPoint: {
         x: sourcePositionInfo.left,
         y: sourcePositionInfo.top,
       },
       horizontalPosition: 'left',
       verticalPosition: 'bottom',
       alignment: 'right',
     };
   } else if (tooltipPosition === tooltipPositions.RIGHT_BOTTOM) {
     return {
       anchorPoint: {
         x: sourcePositionInfo.right,
         y: sourcePositionInfo.bottom,
       },
       horizontalPosition: 'right',
       verticalPosition: 'top',
       alignment: 'left',
     };
   } else if (tooltipPosition === tooltipPositions.LEFT_BOTTOM) {
     return {
       anchorPoint: {
         x: sourcePositionInfo.left,
         y: sourcePositionInfo.bottom,
       },
       horizontalPosition: 'left',
       verticalPosition: 'top',
       alignment: 'right',
     };
   } else if (tooltipPosition === tooltipPositions.LEFT) {
     return {
       anchorPoint: {
         x: sourcePositionInfo.left,
         y:
           sourcePositionInfo.top +
           sourcePositionInfo.height / 2 -
           tooltipSize.height / 2,
       },
       horizontalPosition: 'left',
       verticalPosition: 'bottom',
       alignment: 'right',
     };
   } else if (tooltipPosition === tooltipPositions.RIGHT) {
     return {
       anchorPoint: {
         x: sourcePositionInfo.right,
         y:
           sourcePositionInfo.top +
           sourcePositionInfo.height / 2 -
           tooltipSize.height / 2,
       },
       horizontalPosition: 'right',
       verticalPosition: 'bottom',
       alignment: 'left',
     };
   } else if (tooltipPosition === tooltipPositions.TOP) {
     return {
       anchorPoint: {
         x:
           sourcePositionInfo.left +
           sourcePositionInfo.width / 2 -
           tooltipSize.width / 2,
         y: sourcePositionInfo.top,
       },
       horizontalPosition: 'right',
       verticalPosition: 'top',
       alignment: 'center',
     };
   } else if (tooltipPosition === tooltipPositions.BOTTOM) {
     return {
       anchorPoint: {
         x:
           sourcePositionInfo.left +
           sourcePositionInfo.width / 2 -
           tooltipSize.width / 2,
         y: sourcePositionInfo.bottom,
       },
       horizontalPosition: 'right',
       verticalPosition: 'bottom',
       alignment: 'center',
     };
   }
   invariant(false, `Unexpected tooltip position value: ${tooltipPosition}`);
 }
 
 type GetTooltipPositionStyleParams = {
   +tooltipSourcePosition: ?PositionInfo,
   +tooltipSize: TooltipSize,
   +availablePositions: $ReadOnlyArray<TooltipPosition>,
   +preventDisplayingBelowSource?: boolean,
 };
 
 function getTooltipPositionStyle(
   params: GetTooltipPositionStyleParams,
 ): ?TooltipPositionStyle {
   const {
     tooltipSourcePosition,
     tooltipSize,
     availablePositions,
     preventDisplayingBelowSource,
   } = params;
   if (!tooltipSourcePosition) {
     return undefined;
   }
   const tooltipPosition = findTooltipPosition({
     sourcePositionInfo: tooltipSourcePosition,
     tooltipSize,
     availablePositions,
     defaultPosition: availablePositions[0],
     preventDisplayingBelowSource,
   });
   if (!tooltipPosition) {
     return undefined;
   }
 
   const tooltipPositionStyle = getTooltipStyle({
     tooltipPosition,
     sourcePositionInfo: tooltipSourcePosition,
     tooltipSize,
   });
 
   return tooltipPositionStyle;
 }
 
 type CalculateMessageTooltipSizeArgs = {
   +tooltipLabels: $ReadOnlyArray<string>,
   +timestamp: string,
 };
 
 function calculateMessageTooltipSize({
   tooltipLabels,
   timestamp,
 }: CalculateMessageTooltipSizeArgs): TooltipSize {
   const textWidth =
     calculateMaxTextWidth([...tooltipLabels, timestamp], 14) +
     2 * tooltipLabelStyle.padding;
   const buttonsWidth =
     tooltipLabels.length *
     (tooltipButtonStyle.width +
       tooltipButtonStyle.paddingLeft +
       tooltipButtonStyle.paddingRight);
   const width =
     Math.max(textWidth, buttonsWidth) +
     tooltipStyle.paddingLeft +
     tooltipStyle.paddingRight;
   const height =
     (tooltipLabelStyle.height + 2 * tooltipLabelStyle.padding) * 2 +
     tooltipStyle.rowGap * 2 +
     tooltipButtonStyle.height;
   return {
     width,
     height,
   };
 }
 
+function calculateReactionTooltipSize(
+  usernames: $ReadOnlyArray<string>,
+): TooltipSize {
+  const showMoreTextIsShown = usernames.length > 5;
+  const {
+    maxWidth,
+    maxHeight,
+    paddingLeft,
+    paddingRight,
+    paddingTop,
+    paddingBottom,
+    rowGap,
+  } = reactionTooltipStyle;
+
+  const maxTooltipContentWidth = maxWidth;
+  const maxTooltipContentHeight = maxHeight;
+
+  const usernamesTextWidth = calculateMaxTextWidth(usernames, 14);
+  const seeMoreTextWidth = calculateMaxTextWidth([reactionSeeMoreLabel], 12);
+
+  let textWidth = usernamesTextWidth;
+  if (showMoreTextIsShown) {
+    textWidth = Math.max(usernamesTextWidth, seeMoreTextWidth);
+  }
+
+  const width =
+    Math.min(maxTooltipContentWidth, textWidth) + paddingLeft + paddingRight;
+
+  let height =
+    usernames.length * tooltipLabelStyle.height +
+    (usernames.length - 1) * rowGap;
+
+  if (showMoreTextIsShown) {
+    height = maxTooltipContentHeight;
+  }
+
+  height += paddingTop + paddingBottom;
+
+  return {
+    width,
+    height,
+  };
+}
+
 export {
   findTooltipPosition,
   getTooltipPositionStyle,
   calculateMessageTooltipSize,
+  calculateReactionTooltipSize,
 };